As monkeys-derived Pneumocystis is closely related to P. jirovecii, simian populations should be considered as valuable models for the understanding of the epidemiology of human pneumocystosis. In the present study, the impact of environmental factors on the carriage of Pneumocystis was evaluated in socially organized group of immunocompetent macaques (Macaca fascicularis). The tribe, maintained in partial release at the Primatology Center of Strasbourg in France, comprised 29 animals at the end of the study. From December 2000 to November 2002, deep nasal swab samples were collected monthly from each animal under general anaesthesia. The presence of Pneumocystis DNA was assessed by nested PCR of mtLSU rRNA gene. No case of pneumocystosis was reported during the study. Pneumocystis DNA was detected in 166 out of 481 swab samples examined (34.5%). The number of macaques with detectable Pneumocystis DNA was highly variable from one month to another. However, Pneumocystis carriage was clearly correlated to the mean precipitation rates.