A manipulative field experiment, to investigate the interaction of physiology with ecolgy, is described. Body temperatures of four ecotypes of the lizard Anolis oculatus were studied in their natural habitats on the ecologically diverse island of Dominica. A sample of each ecotypic population was then transferred to one of four 12 times 12 metre experimental enclosures, situated in one of the original habitats. The four in situ populations were found to differ significantly in mean body temperature, and, to a lesser extent, in the degree of thermoregulation. Howerve, no differences were found between enclosed populations. This experiment demonstrates that the apparent differences in the thermal preference of the different ecotypes are attributable solely to the availabiltity of thermal microclimates in the different habitats. The apparent lack of specialization of physiogical traits is surprising in view of the fine-scaled evolutionary adjustment of morphology to geographic variation in local environment that has been recorded in this species, and suggests that more detailed examination of physiology and ecology would be fruitful.