Wild-collected Brachycentrus americanus larvae were subjected to thermal shocks of 30-minute and 60-minute duration in June, September, and December of 1976. Temperatures at collection were 14.7, 10.4, and 1.2 C, respectively. The TL50's for both 30-minute and 60-minute shock durations ranged from 33.3C to 34.C for each month, except for a 30-minute exposure in September, which had a TL50 of 34.6C. Larvae from a December exposure were held for 16 days to observe postexposure behavior. Feeding was reduced 50 percent among specimens exposed to temperatures 1.2C below the 30-minute TL50 and 3.6C below the 60-minute TL50.