In this study, morphological examination and computer-assisted sperm analysis (CASA) of epididymal spermatozoa in non-treated Crj:CD(SD)IGS rats were performed, and the relationship between the data obtained and the retention of step 19 spermatids in Stage IX to XI seminiferous tubules was examined. Retention of step 19 spermatids in Stage IX to XI seminiferous tubules was observed in all 50 untreated males, and the incidence ranged from 3.3% to 100%. Eighteen animals showed a high incidence of retention (74.7 +/- 14.2%, HIR for short), and the others showed a low incidence (24.9 +/- 11.0%, LIR for short). Although the incidence of retention in Stage X and XI seminiferous tubules was very low in LIR males, it was high in HIR males (1.8 +/- 3.0% vs 58.6 +/- 23.2%). Morphological abnormalities of sperms in the caudal region of the epididymis, mainly amorphous head and no head, were more frequently observed in HIR males than in LIR males (36.2 +/- 28.5% vs 1.8 +/- 1.2%). Sperm analysis also revealed some differences between HIR and LIR males: sperm motility in HIR males was severely lower than that in LIR males, and sperm velocity, beat/cross frequency and amplitude of lateral head displacement in HIR males were lower than the corresponding values in LIR males. In summation, retention of step 19 spermatids frequently occurred in the non-treated Crj:CD(SD)IGS males, and a relationship between the retention of these spermatids and sperm abnormalities, such as morphologically abnormal sperms, low motility and other items revealed by sperm analysis (CASA), was suggested.