The present field study was conducted to investigate the effect of environmentally-relevant pollutants on the ultrastructure of the spermatogenic stages and the mature sperm of the mussel, Brachydontes variabilis. For this purpose, two mussel populations were collected from two different sites subjected to different contaminationexposure stresses: a contaminated site (Lake Timsah) and a reference site from Suez Canal. The spermatogenic cells and sperm structure were compared in both populations using light and electron microscopy. Based on TEM observations, the sperm of the investigated mussels is of the primitive type. It is characterized by a head with an ovoid nucleus and a cup-shaped acrosome, a midpiece with a ring of 5 rounded mitochondria encircling two centrioles and a long tail flagellum. The sperm of the present species differs in some structural details from other mytilids. This indicates that sperm morphology is species specific within Mytilidae and thus may be helpful in resolving the taxonomic or phylogenetic relationships within the family. On the other hand, microscopic examination revealed significant adverse effects on testes of the mussels collected from the polluted sites compared to that of the reference site. The most dramatic toxic effect was the presence of some degenerating male follicles. It was also noticed that pollution induced many malformations in the sperm developmental stages including pathological alteration in spermatogonia, spermatocytes and spermatozoa. These results revealed that B. variabilis is sensitive to environmentally relevant levels of pollutants. Based on this finding, it is suggested that this species may be chosen as a model for studying the effects of pollutants on gametogenesis. The adverse toxic effects induced by the environmentally relevant levels of pollutants on the testis of the investigated species may reflect the extent to which the level of pollutants has been reached in Lake Timsah. This may be the main cause of decreasing the success of bivalve communities in the lake in recent years.