Fishes released from constraints of their previous thermal history tend to limit thermal exposure to a narrow range of temperatures. Thermoregulatory behavior (final preferendum) is a stable characteristic of each species studied, suggesting strong regulation by natural selection. However, the adaptive significance of the final preferendum has not been documented for most species. Species with wide geographic distributions, such as bluegill sunfish and largemouth bass exhibit constant laboratory final preferenda regardless of their geographic origin or thermal histories. This apparent evolutionary anomaly of the final preferendum requires further study. It is no longer adequate to describe responses of fish to thermal gradients, rather falsifiable hypotheses concerning adaptive relations between thermoselection, and physiological, biochemical, life-history and genetic correlates must be developed and tested experimentally.