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The importance of thermoregulation as a constraint on behavior has received comparatively little attention in relation to other ecological factors. Despite this, a number of studies suggested that high temperature may represent an important ecological constraint. This paper examines the impact of temperature on activity scheduling in a troop of chacma baboons (Papio hamadryas ursinus) at De Hoop Nature Reserve, South Africa. Once the daily, seasonal, and individual effects were controlled for, the "perceived environmental temperature" (PET), which accounts for the relative contributions of solar radiation, wind speed, and humidity on shade temperature, was a significant constraint on behavior. With high PET, feeding declines, and there is an increase in grooming and particularly resting behavior. Baboons thus engage in more sedentary behaviors as temperature increases, with significantly higher levels of resting and grooming when temperature exceeds the approximate thermal neutral zone for baboons. Seeking shade is an important behavioral response to thermal stress, and PET was a significant determinant of whether an animal was in shade while engaged in either resting or grooming behavior. Furthermore, the proportion of time spent in shade increased across air temperatures that were below, within, and above the thermal neutral zone for baboons. Finally, since resting and grooming are conducted preferentially in certain habitat types, thermoregulatory considerations also impact on patterns of habitat choice and day-journey routes. This suggests that the thermal environment is an ecological variable that should be given greater consideration in future studies of primate behavior. (c) 2005 Wiley-Liss, Inc.