AbstractRegardless of whether fertilization occurs in vivo or in vitro, polyspermic penetration of the human oocyte is a not infrequent cause of reproductive failure. This report describes the occurrence of human eggs fertilized by two spermatozoa and the variable developmental potentials expressed by the resultant embryos. The cellular and subcellular events that characterize the pronuclear and early cleavage stages of preimplantation embryogenesis are discussed with respect to the ability of such embryos to progress to implantation.