Underyearling Bear Lake sculpin exhibit a diel pattern of vertical migration throughout the pelagic region of Bear Lake (Utah-Idaho) until they are approximately 22 mm standard length. Individuals move from the bottom of the lake (5° C) during the day into the water column (13-16° C) at night. The migration, however, is not related to feeding. Although the dominant copepod in the water column, Epischura nevadensis, do undergo a similar diel vertical migration, stomach analysis of juvenile sculpin captured by trawling shows that they do not feed in the water column. Instead, from July through October, their diet is dominated (70-93%) by benthic copepods and ostracods. Also, gut fullness of sculpin increases through the daylight period and decreases through the night, reaching minimum levels just before the dawn descent. Furthermore, feeding trials conducted in the laboratory show that juvenile sculpin feed most efficiently at light intensities found on the bottom (30-60m) of Bear Lake during the day. Feeding rate coefficients increase markedly from 1013 photons m-2S-1 until peaking at intermediate intensities of 1016 photons m-2S-1 and then decline at higher light levels. Although they do not migrate to feed, the movement into the warmer water appears to increase the sculpin's digestion rate, thereby allowing continued feeding during the day. This supports the hypothesis that diel vertical migration in Bear Lake sculpin is a thermoregulatory strategy that increases growth rate.