The body temperatures and activity (time spent in translocation) of Varanus rosenbergi were measured by radio telemetry for a total of 78 lizard-days during summer, winter, and spring. These data, together with laboratory measurements of metabolism, allowed us to estimate the proportion of the total daily energy expended by animals in three types of behavior: resting in their burrows, sitting out of their burrows, and in activity. Although there was a seasonal variation in the total energy expended per day, the proportions spent in the three types of behavior were similar during summer and spring, but a smaller proportion was expended for activity during winter. Nevertheless, the lizards were not completely inactive during winter; they emerged from their burrows and spent several hours above ground, although the mean time spent in activity during this season was only 10.5 min/day compared to 47.6 and 22.2 min/day during summer and spring. We estimate the energy expenditure for the entire population to range from 23.6 kJ ha-1 day-1 during winter to 117.3 kJ ha-1 day-1 during summer. The time spent in activity by this species is strongly seasonal and is positively correlated to the daily solar radiation. However, even during summer, the season of greatest activity, the time spent in activity is not great (mean = 47.6 min/day; range = 12.3-141.3 min/day) compared to the tropical iguanas Conolophus pallidus (mean = 100 min/day) and Cyclura nubila (mean = 92 min/day) and the tropical varanid V. panoptes (mean = 228 min/day during the dry season, its season of greatest activity). Thus, it is inappropriate to generalize all varanid lizards as being extremely active.