Winter-temperature selecton by yellow perch (Perca flavescens) was monitored by radio telemtry near the thermal discharge from a power plant on the Upper Mississippi River. Ten fish were equipped with temperature-sensing radio transmitters. Temperature selection was monitored with an automatic recording system and manually with standard radio frequency receivers. The selected mean winter temperature for all locations was 5.4C; when fish were in the discharge area mean temperature selected was 6.3C. Ninety-one percent of the temperatures were recorded from fish in the discharge bay. Response to temperature varied greatly among individual fish, and they selected significantly lower temperatures in the field than in laboratory experiments.