To provide necessary standardization of procedures for cryopreservation of sperm, a spectrophotomeric method was developed to determine the sperm concentration of diploid and tetraploid Pacific oysters, Crassostrea gigas. Wavelengths of 380, 550, 581 and 780 nm were compared, and 550 and 581 nm were found to be the most sensitive and reliable. A linear relationship between sperm concentration and photometric absorbance was observed for sperm concentrations between 2 × 107 and 2 × 109 cells mL−1. The regression equation for the standard curve at 550 nm for sperm of diploid oysters was Y=−8.528+1.165 log X. The equation for sperm of tetraploid oysters was Y=−8.844+1.236 log X. The equation at 581 nm for sperm of diploid oysters was Y=−8.07+1.104 log X. The equation at 581 nm for sperm of tetraploid oysters was Y=−8.331+1.167 log X. Comparisons derived from the standard curves at 581 nm between observed values and the predicted values indicated good agreement for sperm from diploid (coefficient of determination, r2=0.983) and tetraploid (r2=0.980) oysters.