1. I studied the thermoregulatory behaviour of the diurnal basker Psammodromus algirus (Reptilia, Lacertidae) in the mosaic of sunshade patches of a Mediterranean evergreen forest. 48 behavioural sequences associated with measurements of the thermal environment provided data on the percentages of time spent basking by focal animals, the duration of individual basking periods and the frequency of basking. 2. The percentage of time basking decreased linearly from the onset of the activity period until midday. The average duration of basking periods had a pronounced decrease after 0700 h and remained constant throughout the rest of the morning, whereas the basking rates, that were minimal before 0700 h, reached a peak between 0700 and 0800 h followed by a progressive fall that continued until noon. 3. In order to explain these behavioural patterns, I experimentally determined the effects of environmental variables (air and ground temperature and solar angle) on the heating and cooling rates of captive lizards. The resulting regression models were then used to predict the heat exchange rates of the observed free-ranging animals. 4. The moment at which heating rates should equal and then exceed cooling rates, appeared to set off the behavioural shift towards a larger frequency of shorter basking periods. Under these circumstances, the apparently random movement of a shuttling heliotherm with respect to sun and shade might still be of thermoregulatory significance.