Round spermatids are spermatogenic cells that have just completed meiosis. To discover whether nuclei of these haploid cells are genetically ready for fertilization, nuclei were individually injected into mature hamster oocytes via a microsurgical technique. In the majority of oocytes that were successfully injected and activated, spermatid nuclei transformed into pronuclei and underwent DNA synthesis. Ultimately their chromosomes mingled with ootid chromosomes immediately prior to the first cleavage. Although we could not determine the developmental potential of these zygotes, spermatid nuclei appear capable of participating in syngamy.