- For other uses, see Fly (disambiguation).
Smart Fly is a familiar added in The Binding of Isaac: Rebirth. It is a glowing yellow fly.
Smart Fly is a glowing yellow fly that orbits Isaac and blocks shots. The fly will remain in orbit until Isaac is attacked. After Isaac is attacked, the fly will exit orbit and attack the creature that hit him, along with all other enemies in the room. It attacks similar to a blue fly, but it doesn't die and continues to attack all enemies until they are dead.
- The Smart Fly does damage equal to an attack fly continuously, making it very powerful against slow moving and immobile bosses, killing them very quickly.
- Collecting this familiar and two more fly-based items will turn Isaac into Beelzebub (AKA Lord of the Flies).