The aims of this paper are to compare the thermal ecology of four species of varanid lizards that occupy a range of habitats and climatic regions, and to assess the efficacy of methods for evaluating the extent to which ectothermic animals exploit their thermal environments. Hertz et al. (1993) have proposed several indices of thermoregulation, and these are evaluated with respect to our data from varanid lizards. The thermoregulatory characteristics of three tropical monitor lizards (Varanus panoptes, V. gouldii, and the semiaquatic V. mertensi), and the temperate-zone V. rosenbergi were studied throughout the year. Radiotelemetry was used to measure the body temperatures (Tb's) of free-ranging animals, and microclimatic data were collected to determine the range of possible 7b's that an animal could achieve. Operative temperatures (Te's) were estimated by biophysical models for each set of animal characteristics and microclimatic conditions. The 7b's selected by animals in a laboratory thermal gradient were used to determine the set-point range of Tb's that the animals voluntarily select. Plots that superimpose 7b's, Te's, and the set-point range across the day are extremely useful for describing the thermoregulatory characteristics of ectotherms. These plots can be used to determine the extent to which the animals exploit their thermal environment: we define an index of thermal exploitation (Ex) as the time in which 7b's are within the set-point range, divided by the time available for the animal to have its Tb's within the setpoint range. Only V. mertensi was active throughout the year. In general, during seasons of inactivity, the Tb's of inactive species fell outside the set-point range, but during periods of activity all species selected Tb's within their set-point range. The temperate-zone species (V. rosenbergi) thermoregulates very carefully during periods when environmental conditions allow the animals to attain the set-point range, and V. gouldii also thermoregulates carefully in the wet season. V. mertensi selects 7b's that are significantly lower than the other species both in the field and in the laboratory, and thermoregulatory indices of this species were intermediate relative to the other species. The amount of time spent in locomotion each day was not correlated with the indices of thermoregulation: the most active species, V. panoptes, was, with respect to several indices, the least careful thermoregulator. The type of question that is being addressed, with respect to the interactions between an animal's thermal environment and its thermoregulatory behavior, determines the appropriateness of the various indices of thermoregulation. The Ex index describes the thermoregulatory characteristics of ectotherms in a heterogeneous thermal environment, and in such an environment a large amount of information can easily be interpreted graphically. This index is less useful in a thermally homogeneous environment.