Monstro is one of the bosses that can appear in The Basement and the Burning Basement. He is amongst the most common to fight, particularly because he can appear on later floors as an enemy.
- Hops towards the player.
- Coughs up a large amount of disorganized projectiles. The larger projectiles do one heart of damage while the short-ranged small ones do 1/2 heart.
- Jumps off-screen, aiming to land on top of the player.
- Fires another set of disorganized projectiles after landing.
- Grey: 15% larger and has 25% more HP (312.5). Uses projectile volleys much more often and is unable to jump off-screen.
- Red: Two 25% smaller Monstros, each with 55% less HP than the original one (112.5), replace the normal Monstro.
Monstro spewing out projectiles.
Grey Champion Monstro spewing out projectiles.
Red Champion Monstro spewing out projectiles.
- In the boss room, Monstro can be paired with Black Flies, Attack Flies, Gapers, Hoppers, Frowning Gapers, Mulligans, Cyclopias, or Ministros.
- When Monstro jumps off-screen, he is cured of all status effects.
- Monstro can't hop over Obstacles.
- If Isaac is somewhere that Monstro cannot reach, Monstro will jump in place.
- If Isaac has flight and is over an obstacle, Monstro will eventually be unable to act at all.
- While Monstro can hit Isaac with his tear barrage because of its tear height, it is very unlikely.
- Monstro can rarely spawn in a room with two Blood Donation Machines.
- Monstro appeared in the original The Binding of Isaac.
- Monstro appears to be missing a tooth which happens to be an item.
- Monstro is Monster in Portuguese.
- "Monstro" is a somewhat common mispronunciation of the Spanish word monstruo.