We examined the applicability of the comet assay (single cell gel electrophoresis assay) to estimate the quality of frozen-thawed Pacific oyster (Crassostrea gigas) spermatozoa. Comet assay was performed on semen before and after cryopreservation followed by fluorescent staining with propidium iodide to assess DNA integrity. After cryopreservation, the percentage of spermatozoa with damaged DNA significantly increased, while only about half of the cells displayed intact DNA, even when protected with 10 percent DMSO. All the considered parameters (head length, head area, head intensity, total length, total area, total intensity, tail length percent, tail area percent, and tail intensity percent) were higher than the oyster sperm protected with 10 percent DMSO-artificial sea water after freezing and thawing. Only tail length percent, tail area percent, and tail intensity percent were increased significantly after cryopreservation. The tail length percent was found to be the most sensitive indicator of the cryopreservation-induced DNA damage. Our freeze-thawing procedure significantly affected oyster sperm DNA, as indicated by the reduced fertilization rate when frozen-thawed oyster sperm are used. Irreversible alteration of the genome may prevent fertilization or alter normal embryonic development. This study is the first to demonstrate that the comet assay is an inexpensive, rapid and sensitive method for determining DNA damage in Pacific oyster sperm quality assessments.