Sperm morphology, biochemistry and physiology were compared between Siberian sturgeon Acipenser baerii and sterlet A. ruthenus in the present study. We observed wide morphological variations between spermatozoa of the two species. The average length of the spermatozoon head was 7.01 and 5.14 μm in A. baerii and A. ruthenus, respectively. Significant differences were observed in terms of Na+, K+ and Cl- concentrations in the seminal plasma as well sperm velocity.