Acrosome-reacted CB6F1 mouse spermatozoa with slight flagellar motility were microinjected under the zona pellucida of CB6F1 mouse oocytes. Electron microscopy revealed the presence of swollen and decondensed sperm heads in the oocyte cytoplasm. Sixty-one percent of the microinjected oocytes reached a morphologically apparent two-cell stage, but chromosomal analysis demonstrated only haploid chromosomal complements in all cases. The exposure of microinjected oocytes to suspensions of spermatozoa of mice homozygous for a 2,4 reciprocal translocation resulted in normal fertilization and embryonic development with a maternally as well as a paternally derived haploid genome. Identical results were obtained with oocytes microinjected with medium and subjected to in vitro fertilization thereafter. Thus it can be suggested that the microinjected spermatozoa with insufficient flagellar motility are incorporated into the oocyte cytoplasm by phagocytosis. These spermatozoa do not induce a polyspermy block but induce the oocyte to parthenogenetic development.