Basement

The Basement is one of the environments in Chapter 1. Unlocked at the beginning of the game, it is considered the easiest of the environments in the game. The theme of the Basement is flies and poop.

The Basement's harder unlockable variants are the Cellar and the Burning Basement.

After A Secret Exit is unlocked, a locked door similar to the one leading to a Treasure Room will appear in this floor's Boss Room upon defeating the boss, which leads to Downpour / Dross. Like a Treasure Room, this door requires a key to be unlocked. Unlike Angel Rooms and Devil Rooms, this door never disappears.

Monsters

Entity	ID	Description
Clotty	15.0	Randomly wanders around the room, firing 4 blood shots in the cardinal directions.
Clot	15.1	Randomly wanders around the room, firing 4 tar shots in the ordinal directions.
I.Blob	15.2	Randomly wanders around the room, firing 8 shots in all directions.
Dip	217.0	Slowly dashes in random directions, dealing contact damage.
Squirt	220.0	Dashes towards Isaac and deals contact damage. Spawns 2 Dips upon death.
Embryo	77.0	Aimlessly hops around the room, dealing contact damage.
Fatty	208.0	Slowly walks towards Isaac. Occasionally farts, which pushes Isaac and adjacent enemies away.
Pale Fatty	208.1	Slowly walks towards Isaac. Occasionally farts and shoots 6 blood shots in all directions. Has a chance to turn into a Blubber upon death.
Blubber	210.0	Walks around aimlessly and randomly spews a cluster of blood shots in all directions. Has a chance to turn into a Half Sack when killed.
Fly	13.0	Hovers in place. Does not deal contact damage and is harmless, but can turn into Pooters if they stay around Poop.
Attack Fly	18.0	Red fly that follows Isaac and deals contact damage.
Ring Fly	222.0	Gathers with any other Ring Flies in the room. These flies orbit around themselves while following Isaac. Deals contact damage.
Moter	80.0	Follows Isaac, similar to Attack Flies. Deals contact damage. Splits into 2 Attack Flies upon death.
Pooter	14.0	Slowly drifts toward Isaac, occasionally firing a single blood shot at him. Does not deal contact damage.
Super Pooter	14.1	Slowly drifts toward Isaac, occasionally firing two blood shots in a V-formation at him. Does not deal contact damage.
Level 2 Fly	214.0	Flies toward Isaac erratically, dealing contact damage.
Full Fly	249.0	Flies toward Isaac erratically, dealing contact damage. Releases 6 bullets in all directions upon death.
Sucker	61.0	Flies slowly towards Isaac, exploding into 4 shots in the cardinal directions upon death.
Gaper	10.1	Walks towards Isaac, dealing contact damage. Usually turns into a Pacer or a Gusher upon death.
Frowning Gaper	10.0	Walks towards Isaac, dealing contact damage. They start out with their eyes closed, but when they gain line of sight with Isaac, their eyes open and they permanently move faster. Usually turns into a Pacer or with a small chance a Gusher upon death.
Skinny	226.0	Slowly walks towards Isaac, leaving short-lasting Red Creep behind itself.
Hopper	29.0	Slowly hops around the room, doing contact damage. Can turn into a Flaming Hopper by jumping into Fire Places or other fires.
Flaming Hopper	54.0	Quickly hops around the room, doing contact damage. Covers a larger distance than the regular Hopper.
Horf	12.0	Stationary enemy that fires a medium-ranged blood shot at Isaac when he gets in range of them.
Psychic Horf	248.0	Stationary enemy that fires a homing shot at Isaac when he gets in range of them.
Mulligan	16.0	Walks around the room, avoiding Isaac. Upon death, spawns several flies, which can be Flies, Attack Flies, and/or Pooters. Can self-detonate and launch four projectiles in the cardinal directions while still releasing flies.
Mulligoon	16.1	Walks around the room, avoiding Isaac. Upon death, spawns a Troll Bomb and fires four shots in the ordinal directions. Can self-detonate, spawn a Troll bomb, and launch eight projectiles in all directions.
Mulliboom	16.2	Chases Isaac and explodes upon death or upon touching Isaac. The explosion deals a half-heart of damage instead of the regular full heart.
Hive	22.0	Wanders around the room, avoiding Isaac and spawning Attack Flies if he gets close. Spawns a large number of flies on death, which can be Flies, Attack Flies, and/or Pooters.
Nest	205.0	Wanders around the room, avoiding Isaac and spawning Spiders if he gets close. Spawns a Big Spider or Trite upon death.
Nerve Ending	231.0	Static enemy which acts as a meat shield.
Pacer	11.1	Walks aimlessly around the room.
Gusher	11.0	Walks aimlessly around the room, occasionally firing a short-ranged blood shot.
Round Worm	244.0	Pops out of the ground in random locations and spits a blood shot at Isaac if he is in sight.
Spider	85.0	Erratically moves around the room. If Isaac gets close to it, it chases him, trying to deal contact damage.
Level 2 Spider	215.0	Crawls around the room erratically, following Isaac and dealing contact damage. Can jump small distances.
Ticking Spider	250.0	Crawls around the room erratically, following Isaac and dealing contact damage. Can jump small distances. Explodes upon death.
Baby Long Legs	206.0	Runs away from Isaac and frequently spawns Spiders.
Small Baby Long Legs	206.1	Runs away from Isaac and occasionally spawns Sacks.
Crazy Long Legs	207.0	Chases Isaac, dealing touch damage. Occasionally stops to spit 8 blood shots in all directions.
Small Crazy Long Legs	207.1	Chases Isaac, dealing touch damage. Occasionally stops to spit 4 blood shots in the diagonal directions.
Wall Creep	240.0	Crawls along walls to try to line up with Isaac and shoots three blood shots in a line.
Wall Hugger	218.0	Moves around the room like Guts, damaging Isaac by contact. Cannot be destroyed.
Brownie Corn	217.2	Quickly dashes in random directions, dealing contact damage.
Dart Fly	256.0	Flies towards Isaac and attempts to predict his movements. Deals contact damage.
Swarm	281.0	Follows Isaac in large, tightly-clustered groups and deals contact damage.
Cyclopia	284.0	Walks towards Isaac, dealing contact damage and leaving behind a short-lasting trail of damaging Red Creep. Occasionally shoots a blood shot towards Isaac.
Blue Gaper	297.0	Walks towards Isaac, dealing contact damage.
Greed Gaper	299.0	Walks towards Isaac, dealing contact damage.
Conjoined Fatty	257.0	Slowly walks towards Isaac. Occasionally pauses to fart out a line of damaging Green Creep towards Isaac. Chance to turn into a regular Fatty upon death.
Suction Pitfall	291.1	Invulnerable and stationary enemy which sucks Isaac towards itself. Isaac falls into it when walked over, taking damage and popping back out after a short delay.
Rag Man's Ragling	246.1	Quickly leaps towards Isaac and deals contact damage. Randomly shoots two purple shots that orbit around it.
Roundy	276.0	Pops out of the ground in random locations, creating a small rock wave. Fires an explosive Ipecac shot at Isaac if he is in sight.
Ulcer	289.0	Pops out of the ground in random locations and spawns a Dart Fly if Isaac is in sight. Fires a blood shot after spawning 6 Dart Flies.
Stoney	302.0	Chases Isaac and blocks tears, but does not deal contact damage. Temporarily pauses in exhaustion after a certain amount of time. Invulnerable; dies when the room is cleared.
Ministro	305.0	Hops around randomly and hop towards Isaac when he gets close, occasionally spitting out a cluster of disorganized shots.
Portal	306.0	Spawns various enemies from the current floor until it either vanishes or is killed.
Army Fly	868.0	Chases Isaac, moving along the cardinal directions. Forms a line with other Army Flies in the room. Deals contact damage.
Bulb	61.5	Flies in erratic, angled movements toward Isaac if his active item has charges; on contact, they drain up to 2 charges, but never deal contact damage. If he has no active item, or no charges, they wander around the room aimlessly.
Swarm Spider	884.0	Very erratically hops toward Isaac in large groups, dealing contact damage.
Needle	881.0	Pops out of the ground and lunges towards Isaac, dealing contact damage.