While the fact of behavioral thermoregulation in fishes has been well documented, mechanisms remain obscure. A successfully thermoregulating fish moves through its habitat in such a way as to maximize time spent at temperatures favorable for the joint conduct of its life processes. The dynamics of its distribution must be related mechanistically to thermal structure of the habitat. Logic and research literature suggest that appropriate distributional mechanisms fall into two classes, predictive and reactive. Predictive thermoregulation comprises directed movements to a subset of habitat within which the fish “expects,” on the basis of individual or evolutionary experience, to find acceptable temperatures. Reactive thermoregulation involves a sequence of undirected movements that are biased by recent thermal experience so that net movement is towards the thermal preferendum. Exploration of possible reactive mechanisms via computer simulation indicates that successful thermoregulation could be accomplished by a fish that tends to increase its rate of turning whenever recent experience implies worsening thermal conditions.