AbstractPhagocytosis of supernumerary spermatozoa contained within the perivitelline space was observed in 7 of 28 two‐cell mouse embryos cultured for 45 hours post‐insemination (approximately 20–24 hours after cleavage). The spermatozoa were incarcerated as a result of elevations of the blastomere cytoplasm which gradually surrounded the sperm, overlapped and fused, thus forming a typical phagocytic vacuole. Phagocytosis was not observed in two‐cell embryos cultured for less than 20–24 hours after cleavage; this indicates that the blastomeres of two‐cell mouse embryos in vitro require approximately 24 hours to develop one of the characteristics of somatic cells, i.e., the ability to recognize and phagocytize forei