Some biological aspects of semen were investigated in the Persian sturgeon, Acipenser persicus, by determination of seminal plasma indices (ionic composition and osmolality), sperm density and their relationships with sperm motility. The osmolality of seminal plasma ranged from 42.00 to 111.00 mOsmol Kg-1. The concentrations of Sodium (Na+), Potassium (K+), Chloride (Cl-), Calcium (Ca2+) and Magnesium (Mg2+) ions were 62.44±6.82, 6.92±0.88, 21.11±5.41, 0.79± 0.03 and 0.52±0.03mM L-1, respectively. The sperm density was 8.34±1.38×109 spz/ml. The Sodium/Potassium and Calcium/Potassium ratios were 9.02 and 0.13, respectively. The following significant positive correlations were found between seminal plasma indices; osmolality - Cl- (r2= 0.492, P<0.05), osmolality - Na+ (r2= 0.905, P<0.001) and Na+ - Cl- (r2= 0.584, P<0.05). There were also no significant correlations between ionic composition and osmolality of the seminal plasma and sperm motility (P>0.05). No relationship was observed between sperm density and its motility at the concentrations tested (r2= 0.015, P>0.05). The ionic composition and osmolality of seminal plasma reveals an inter-species specific as well high secretory activity of sperm duct. The clear differences observed in A. persicus should be considered when cryopreservation methods for sperm are developed in Acipenseridae species.