AbstractGenetic variation in spermatozoa was used to examine mechanisms important for fertilization in the mouse. A significantly greater proportion of cauda epididymal sperm from C57BL/6 (inbred) males were motile than from random‐bred (CFW) males. Random‐bred sperm, however, were able to fertilize a significantly greater percentage of eggs in vitro than were inbred sperm. When sperm of these two genotypes were used for insemination in vivo, and the penetrated eggs cultured through the first cleavage, the levels of cleavage were similar, suggesting that neither levels of sperm motility nor sperm penetration in vitro accurately reflect the ability of the same sperm populations to penetrate eggs in v