14The vaginal microbiome of nonhuman primates (NHPs) differs substantially from humans 15 in terms of Lactobacillus abundance, overall taxonomic diversity, and vaginal pH. Given 16 these differences, it remains unclear in what way the NHP genital microbiome protects 17 against pathogens, in particular sexually transmitted diseases. Considering the effect that 18 microbiome variations can have on disease acquisition and outcome, we examined 19 endogenous and exogenous factors that influence the urogenital microbiome of captive 20 rhesus monkeys. The male urethral (n=37) and vaginal (n=194) microbiome of 11 21 breeding groups were examined in a cross-sectional study. During lactation and 22 menstruation, the vaginal microbiome becomes significantly more diverse and more 23 similar to the microbes observed in the male urethra. Group association and cage-mate 24 (sexual partners) relationships were additionally associated with significant differences in 25 the urogenital microbiome. Our results demonstrate that microbiome considerations are 26 necessary in order to make informed selection of NHPs as translational animal models. 27