Flies buzz around the room randomly, often moving away from the player. They cannot damage Isaac at all, even by touch, making them completely harmless. However, they can turn into Pooters if they fly around poop for too long or come in contact with a golden poop.
Attack Fly
Attack Flies are red and black flashing flies that will follow Isaac relentlessly until killed. Their only way of damaging Isaac is by contact, and they move rather slowly, making them easy to kill. However, they can be dangerous when found in groups, as they can swarm around Isaac quickly.
Attack Flies turn into regular Black Flies if Isaac has 
Skatole. They take a small amount of time to convert into Black Flies, so it is possible for Isaac to be damaged if the player is not careful.
Notes
- The Duke of Flies and other bosses can spawn larger versions of these flies which have 10 Base HP.
Dart Fly
Dart Flies are similar to regular attack flies, but attempt to mimic Isaac's movements. Their only way of damaging Isaac is by contact, and they can move fairly quickly, making them potentially dangerous when found in groups.
Swarm Fly
Swarm Flies are similar to regular attack flies but appear in tightly-packed clusters. Their only way of damaging Isaac is by contact, and although they move fairly slowly, they can be dangerous by simply outnumbering Isaac.
Notes
- If a Swarm Fly is separated from its group due to being petrified, it quickly moves back to its group once unpetrified.
Hush Fly
Hush Flies are spawned by Hush and behave somewhat similarly to Ring Flies, flying as towards Isaac in certain patterns. Unlike Ring Flies, they can form other shapes such as polygons and can also orbit Hush.
Hush Flies have a brief period of invincibility when Hush spawns them, making them appear to have more health than they have.
Hush Flies are not affected by the Beelzebub Transformation.
Hush Flies can also rarely spawn outside of the Hush fight in rooms.
Moter
Moters appear as two Attack Flies joined together. They behave similarly to regular Attack Flies but split into two Attack Flies upon being killed. Their split is very wide and unpredictable and can easily shave off half a heart. It is recommended to kill faster flies and other small monsters before it as the two flies it spawns are more dangerous in many ways. Running between them just as they split is also a very good way to dodge.
- If Isaac has
Skatole, or uses a bomb, killing a Moter will not make them split into two Attack Flies.
Eternal Fly
The Eternal Fly is a white-colored fly that can be found on any floor in the game. They usually circle corny poop or enemies like Holy Leeches and the Hanger and will not stop circling them until their hosts are destroyed. Before that, they will deal damage to Isaac if he touches them, and they're completely immune to all forms of damage. When their hosts are destroyed, they convert into Attack Flies.
- If Eternal Flies are circling Corny Poop and there are no other enemies around, you can leave the room at will. This is true even if they have already been converted into large attack flies: they will simply disappear when you return into the room.
Notes
- Eternal Flies can be killed by Chaos Card, Little Horn, and Euthanasia.
Ring Fly
Ring Flies are a new addition to Rebirth. They are pink flies that, upon entering the room, will cluster together into a circular formation and orbit rapidly while moving towards Isaac. They will hold formation even when you destroy all but one fly. The maximum amount of flies per formation is 10. These flies are fast and take up a large amount of space, so it may be recommended to hold these on higher danger priority.
- Ring Flies are not affected by Strange Attractor and will continue to stay within their current circular formation.
Sucker
The Sucker has a greater amount of health than a regular fly and slowly moves towards Isaac. Upon death, it shoots 4 bullets in all cardinal directions. Suckers are unaffected by Skatole, so they're always hostile.
Despite their simplistic behavior, Suckers can prove rather dangerous at late levels. This is because oftentimes they'll be positioned in areas containing hazards or the player may have familiars that attack the Suckers regardless of the player's intention. As a result, the low health that the Suckers possess can be used to give the player little time to react when they die quickly and shoot their projectiles outwards.
Spit
The Spit looks like a green version of the Sucker, and it shoots one explosive green bullet at the player upon death. The shot behaves exactly like 
Ipecac. Spits are unaffected by Skatole, so they're always hostile. Like most enemies that possess explosive attacks, Spits can be used to bomb tinted rocks or open Secret Rooms.
Soul Sucker
Soul Suckers were added in Booster Pack #4.
The Soul Sucker looks like a bloated, red-spotted version of the Sucker. Upon death, a swirling red vortex appears where it died, which then fires four 
Brimstone beams in the four cardinal directions after a short delay.
The appearance of the vortex resembles the same vortex that is spawned whenever the player fires with Brimstone and 
Anti-Gravity.
Notes
- The Brimstone lasers from the Soul Suckers can damage other enemies, including other Soul Suckers. You should play carefully when facing multiple Soul Suckers, as their vortex attacks could corner you if you're not careful.
Trivia
- The Black Fly has the lowest health of any enemy in the game, tied with the Corn.
- There is a fly enemy (popularly known as the Ball of Flies) in the Eternal Edition of the original Binding of Isaac that is a ball of 4 attack flies, resembling 2 Moters stuck to each other. This enemy does not appear in Rebirth, likely due to being added in said Eternal Edition update that was added a while after the release of Rebirth.
- Suckers seem to be based on mosquitoes, midge-like flies that suck the blood of other animals, as they seem to share a small proboscis and blood sac.
- In a Q&A Reddit thread by Edmund McMillen, Edmund McMillen confirmed that the species of flies in the Binding of Isaac are house flies and botflies.
Gallery
Attack Fly on Isaac's Last Will.
Concept art for the Soul Sucker.
 The Binding of Isaac: Rebirth
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