Any hitherto available technique used to detect the presence of Wolbachia, a rickettsia-like endosymbiont of Drosophila simulans, i.e., crossing with tester strains, electron microscope observations, DAPI-staining of embryo, or amplification of specific sequences by PCR are notably time-consuming techniques. Staining sperm cysts with the fluorochrome DAPI is here shown to be a fast and reliable technique to detect the symbiont and evaluate the level of infection of individual males. Wolbachia cells are abundant in sperm cysts and make up a mass which is eliminated in the waste bag during spermatogenesis. Such a mass was absent in all those strains known to be aposymbiotic. In infected strains, tetracycline treatment led to the disappearance of the symbionts within sperm cysts. In aging males, a decrease in the proportion of infected cysts were observed. There was however no apparent change in the number of symbionts in the cysts which remained infected. The reduction of reproductive incompatibility, which is observed in older males, may be explained by this process, i.e., a decrease in the frequency of infected cysts.