Observations of nonhuman primate long bone microanatomy were undertaken with the explicit goal of recording intra- and interspecific variation in the percentage of osteonal bone. One-hundred and eight specimens with age, sex, body size, and positional behavior data representing 7 species (Galago seneqalensis, Otolemur crassicaudatus, Macaca fascicularis, Macaca mulatta, Macaca arctoides, Erythrocebus patas, and Cercopithecus aethiops) were examined. Humeral and femoral shafts were examined histologically at the proximal, midshaft, and distal cross sections. Furthermore, the humeral and femoral midshaft sections were histologically examined at the anterior, posterior, medial, and lateral quadrants. Each cross section and quadrant were read specifically for percentages of osteonal bone. Data on cortical bone area and cortical thickness were also recorded for each section. Descriptive statistics, analysis of variance, and Model II regression (maximum likelihood estimation) were used to test for intra- and interspecific variation in osteonal bone for this population of primates. In general, several points concerning primate skeletal microanatomy have been revealed: 1) humeral and femoral microanatomy do not mirror each other in osteonal pattern, suggesting that different factors affect the bone microstructure of these limb bones, 2) primate species with different positional behavior express different distributions of osteonal bone and these differences reflect body size and mechanical usage, 3) after sexual maturation age does not affect intraspecific variations in the percentage of osteonal bone, 4) osteon area scales positively allometrically on body size (cortical area) suggesting that the rate of osteon area increases faster than the rate of increase in body size among primate species. Other conclusions have been drawn from this study, the most important point of which is that mechanical loading plays a large role in stimulating secondary osteon formation and that osteonal bone distribution reflects positional behavior and body size. This has profound implications for how future primate histological research can be conducted. The results of this study also have implications for the application of histological analysis to extinct primate specimens. Overall, this information will give primatologists additional important tools to be used in examining skeletal form as it relates to function in both living and extant primate species.