Three lots of 6-day-old to IPh-month-old channel catfish from a common initial source but reared under different thermal conditions were utilized to investigate several heat-tolerance relationships. During the experimental period the fish were held in constant-temperature tanks and samples of fish were subjected to a range of sub-lethal and lethal temperatures in test baths. The results were studied by plotting time to death of individual fish and means of test samples on semi-log paper, probability paper in some instances, and subjecting pertinent data to statistical analysis. Fish acclimated to 26.0, 30.0, and 34.0C had upper lethal temperatures of approximately 36.6, 37.3, and 37.8C respectively. Differences in resistance times were related to age but not to size. The results of prolonged exposure to "semi-lethal" test temperatures indicated that channel catfish may die from the direct results of heat for at least 13 %, days after initial exposure and at slightly lower test temperatures die from the indirect results of hellit, due to an increased rate of metabolism resulting in starvation. The data indicated the presence of approximately six "lethal effects" that separated the fish into physiological populations on the basis of their response to lethal temperatures.