The changes in population trends of fish in the Wabash River resulting from the introduction of thermal effluents were examined for a 14-year period from the standpoint of whether the observed changes could have been predicted from the results of laboratory determinations of thermal tolerance alone. It was concluded that gross changes in community structure could have been predicted but the degree to which most species populations and the community as a whole were influenced would require further detailed information about (1) the chemical and physical characteristics of the environment and (2) the attributes of the resident biota in terms of behavioral responses, physiological capabilities, and life history requirements.