Temperature tolerance quantified as CTminima and CTmaxima of channel catfish (Ictalurus punctatus), largemouth bass (Micropterus salmoides) and rainbow trout (Oncorhynchus mykiss) periodically measured during 32 days of exposure to a 10 °C diel thermoperiod were compared to those determined at constant acclimation temperatures equal to the minimum, midpoint and maximum of each species' thermoperiod. The diel thermoperiod for channel catfish and largemouth bass extended from 20 to 30 °C while rainbow trout were cycled between 10 and 20 °C. A hypothesis that the zone of temperature tolerance would be enlarged during exposure to a 10 °C thermoperiod was tested. Mean CTmaxima and CTminima during cycling ranged from 38.5 to 39.6 °C and 6.1 to 6.6 °C (channel catfish), 35.6 to 37.3 °C and 5.9 to 7.7 °C (largemouth bass) and 27.3 to 29.3 °C and 0.1 to 1.4 °C (rainbow trout). Our data indicate that with one exception both upper and lower temperature tolerance for each of these three species exposed to the diel temperature cycle were more similar to those of fish acclimated to a constant temperature near the midpoint of the thermoperiod than to either the minimum or maximum temperature of the cycle. The exception occurred in rainbow trout where the mean CTmaximum during cycling was equivalent to an acclimation temperature midway between the mean and peak acclimation temperature. Furthermore, in all three species the temperature tolerance scope (CTmaximum - CTminimum) was less in fish exposed to the thermoperiod used in this research. The temperature tolerance ability of any of the three test species was not enhanced.