The 24-hour activity patterns of young and adult black bullheads have been investigated in a pool of a Wisconsin stream as well as in the laboratory. The young were found to exhibit two primary feeding periods in nature, one around dawn and the other around dusk. The young swim actively in schools during most of the daylight hours with an apparent depression of activity occurring around midday. They appear to be relatively inactive at night. Adults apparently remain inactive in the weed beds during the daylight hours, but they move around extensively at night. Trapping evidence suggests that the adults tend to forsake the pool during the early hours of darkness and return shortly before dawn. Feeding of the adults takes place after dark, but its exact timing is unknown. Behavior of the bullheads appears to be controlled by a combination of internal and external factors. Internal factors include hunger and “social appetite.” External factors include food, other bullheads, and possibly light.