Largemouth bass Micropterus salmoides were reared over their first 8 weeks of free-swimming life in uncontaminated control water or in water containing one of five concentrations of pentachlorophenol (PCP) ranging from 1.6 to 88 μg/L. Over the final 3 weeks of the study, fish reared in concentrations of 67 and 88 μg PCP/L performed significantly fewer feeding acts (orientations, bites) and had a lower rate of prey capture than did control fish. However, fish in high concentrations spent significantly more time swimming than did control fish, which indicated that exposure to PCP made them hyperactive. By inhibiting energy intake while inducing higher energy expenditures, PCP may reduce survival of young largemouth bass over the winter.