Examination of sperm head morphology is one of the requisite tests of the functional capacity of semen in reproduction. In the present study, genetic effects on morphological sperm head abnormalities in mice were investigated. The frequency of abnormal spermatozoa was determined in 17 inbred mouse strains and it was found that strain B10.M had the highest frequency of abnormal spermatozoa (44.7%). Segregation analysis was then used to show that the abnormal sperm phenotype in B10.M mice was inherited. The results indicated that this sperm abnormality was controlled by two distinct recessive alleles. It is proposed that the high frequency of the heritable abnormal sperm phenotype in the mouse B10.M strain explains the subfertility of this strain, as evidenced by its reduced litter size.