The estuarine goby Gillichthys mirabilis behaviorally thermoregulates when placed in a laboratory temperature gradient. Avoidance of temperatures above 23°C is the most evident component of this thermotaxis. Temperature preferences of non-acclimated fish do not vary significantly as a function of season; nor does temperature acclimation alter the preferendum. Varying the temperature range of the gradient apparently modifies thermotaxis. Negative phototaxis evidenced by this species is subjugated by thermal preferences. Temperature preference does not vary diurnally. The heat resistance of this species was determined; there is no apparent relationship between heat resistance and temperature preference.