Brown bullheads acclimated to temperatures between 3.5 and 28 C and tested in a thermal gradient trough usually preferred water temperatures above their respective acclimation temperatures. Fish tended to gravitate toward higher temperatures the longer they remained in the gradient; final preferendum was 27.3 C. Laboratory results are consistent with observed movements of Connecticut River brown bullheads into the thermal discharge canal of a power plant during autumn. It appears from both laboratory and field studies that a “temperature trap” keeps bullheads in the warm discharge canal during winter. However, brown bullheads leave the canal in spring sooner than would be predicted from temperature preference data alone.