A common method for collecting behavioral data is through direct observations. However, there is very little information available on how a human observer affects the behavior of the animals being observed. This study assesses the effects of a human observer on the behavior of captive nonhuman primates. The subjects were 19 singly housed baboons (nine male, 10 female) and 20 singly housed rhesus macaques (10 male, 10 female) that were not habituated to the presence of an observer. Four 30-min observations were conducted on each animal. Two observations were conducted with an observer present ("present" condition), while the remaining two observations had no observer present ("absent" condition). All observations were recorded with a video camera and were balanced for time of day, with one of each type of observation taking place in the morning and afternoon. In the presence of an observer, appetitive behavior was significantly reduced in both species [F(1,35) = 8.22, P < 0.01]. When an observer was present, females of both species also rested more and performed fewer manipulative behaviors than males [rest: F(1,35) = 7.10, P < 0.05; manipulative: F(1,35) = 6.66, P < 0.05]. Likewise, macaques rested significantly more [F(1,35) = 11.62, P < 0.005] and exhibited fewer manipulative behaviors in the presence of an observer [F(1,35) = 11.06, P < 0.005], while baboons showed no change. Female macaques showed the greatest decrease in activity while an observer was present [F(1,35) = 4.22, P <0.05]. Based on these results, the presence of a human observer does appear to affect the behavior of unhabituated, singly housed baboons and macaques, but the effect differs by both sex and species.