We have previously reported that epididymal spermatozoa from mice carrying a tw32 haplotype have a lower net velocity than do spermatozoa from congenic +/+ males. Using a test for nonprogressive motility based on the inability of a spermatazoon to leave a cube 200 micron on each side in less than 4 seconds, the frequency of nonprogressively motile sperm in these populations now has been determined to be above 90% after 2 to 5 hours of incubation in vitro. The nonprogressive motility never appears in sperm populations incubated in media with less than 0.1 mM Ca2+, although normal motility is well-maintained. Nonprogressive motility was also observed at low frequencies in uterine sperm populations, and at higher frequencies among uterotubal sperm from both +/+ and tw32/+ males. These observations suggest that nonprogressive motility may be a component of normal sperm function, either during sperm transport within the oviduct, or during fertilization.