"Growth responses of the white sucker, Catostomus commersoni. were examined in relation to the influence of temperature, body size, season, day length, light intensity, food ration level and food quality. Sucker growth was maximum at a temperature range of 19-26ÀC, depending upon experimental conditions. Fish reared under low light intensities grew an average 43% faster than those reared under unshaded conditions. Growth on various diets was best on live tubificid worms presented over sand substrate > tubificids (no soil substrate) > frozen Daphnia > Oregon Moist pellets > Glencoe Mills pellets. The optimum temperature for growth on excess rations of live tubificids was 25ÀC and was 19ÀC on restricted rations (1.5% fish body dry weight). Maximum specific growth rate decreased nearly four-fold over a size range of 12 to 175 g, but no difference in optimum temperatures was found. Fish of the same approximate size grew twice the rate in the spring as compared to other times of the year. Photoperiod showed little influence on growth rate, but fish exposed to shorter day length showed a marked increase in time to achieve a maximum growth rate. The ultimate upper incipient lethal temperature (UUILT), determined by slowly increasing (0.5ÀC/day) acclimation temperature to death, was 32.5ÀC for juvenile white suckers and 31.5ÀC for adults. The UUILT was 2-3ÀC higher than the upper lethal temperatures measured by the classical approach involving the direct transfer technique."--P. .