The thermal relations, diel, and annual activity and microhabitat use of Lacerta agilis were studied in the Pyrenean isolated range of this species. Although, this heliothermic lizard demonstrated thermoregulatory capability, thermal constraints attributable to the mountain climate were observed. The activity pattern was that typical of cold temperate lizards but the beginning of activity was delayed in comparison with lowland populations. The reproductive cycle determined different activity patterns for males and females. The ontogenetic and seasonal changes in microhabitat use detected suggest influence of body size, reproductive condition in adults, and interference with other individuals in juveniles.