Abstract: To study the impacts of environmentally hazardous organophosphate, Diazinon (DZN), on male reproductive system of biologically, nutritionally and economically pivotal Cyprinidae, Caspian kutums (Rutilus frisii kutum), the in vitro effects of DZN on spermatogenesis and sperm motility of maturing and mature fish investigated. The fish testes were exposed to sub-lethal concentrations of DZN, 0.01, 0.1 and 0.2 mg L-1, using in vitro tissue culture, for 3 and 6 days. In other experiment, percentage of motile spermatozoa and its total duration were measured in solutions containing 0.01, 0.1, 0.2 and 0.5 mg L-1 of DZN. Adverse effects on testes were more apparent with the increase of DZN concentration. At concentration of 0.1 and 0.2 mg L-1 of DNZ a remarkable necrotic germ cells were observed. Compared to control groups, the mean size of Spermatogonia (SPG) and Spermatocycle (SPC) significantly decreased (p<0.001) in dose-dependent manner. With the increase of DZN concentration, the number of SPCs and Spermatids (ST) decreased (p<0.001) but the number of second SPG, increased (p<0.05). With the increase of DZN concentration, percentage of motile spermatozoa and total duration of spermatozoa significantly decreased (p<0.05) but there was no significant difference between 0.01, 0.1, 0.2 mg L-1 (p>0.05). Our study supports the detrimental impacts that pollution of aquatic environment with very low levels of DZN has adverse effects on male testes and sperm quality in the Caspian kutum.