The lacertid lizard Podarcis hispanica atrata maintained remarkably constant body temperatures (Tb) throughout its daily activity period in early autumn, despite significant variability in available operative temperatures (T[e]). Lizard T[b]S were substantially closer to the species' selected temperature range (the target T[b]S for thermoregulation, estimated by the central 80% of the temperatures selected in a laboratory photothermal gradient) than were a random sample of T[e]S, indicating that this population thermoregulates both accurately and effectively. We compared observed behaviors and T[b]S to the predictions of two hypotheses. The no thermoregulation null hypothesis predicted that lizard activity and microsite use would be unaffected by thermal constraints. The only thermoregulation hypothesis predicted that lizards would be active only at those times and in those places where T[e]S fell within the selected temperature range. To evaluate the contributions of various behavioral adjustments (activity times, use of microhabitats and sun-shade patches, basking, and shuttling) to temperature regulation, we combined results from direct behavioral observations with information on the T[e]S available at different times of day and in different microsites (combinations of structural microhabitats and sun-shade patches). Lizard activity patterns did not match the predictions of either hypothesis, but the observed pattern of activity produced T[b]S that were, on average, 1.0°C closer to the selected temperature range than were T[b]S predicted by the no thermoregulation hypothesis. The lizards' use of microsites (combinations of structural microhabitats and sun-shade patches) was more similar to predictions of the no thermoregulation hypothesis than to those of the only thermoregulation hypothesis. Nevertheless, the lizards' use of specific microsites produced T[b]S that were, on average, 1.9°C closer to the selected temperature range than were randomly available T[e]S. The relative contributions of different behavioral mechanisms varied with time of day. In the early morning, low movement rates, selection of sunny microsites, and most notably, adoption of a basking posture contributed to temperature regulation. In the late morning and early afternoon, no thermoregulatory behaviors were necessary, as indicated by randomly positioned models that achieved mean T[e]S within the selected temperature range. At midday, frequent movements between sunny and shaded microsites probably reduced heat loads on lizards, preventing T[b]S from exceeding the selected temperature range. In the late afternoon, nonrandom use of microsites and basking behavior appeared to contribute equally to the elevation of T[b]S above randomly sampled T[e]S.