Samples of juvenile and adult yellow perch (Perca flavescens) were obtained from the same habitat and at the same time from two separate water sources to study the differences, if any, in their preferred temperatures.The fish, acclimated to 24 C, were subjected to a vertical temperature gradient. Juveniles selected temperatures in the range 20.0–23.3 C and adults in the range 17.6–20.1 C. It is concluded that age plays an important role in temperature selection in this species.