Studies were made on the contribution of haploid-derived parthenogenetic cells to haploid parthenogenetic ↔ fertilized chimeric embryos on day 9 and 10 of pregnancy. In most cases, the contribution of haploid-derived parthenogenetic cells to embryonic tissues was higher than that to extraembryonic tissues. The contribution of haploid-derived cells to embryonic tissues of some chimeras was more than 90%. Chromosomal analysis showed that actively dividing cells in most chimeric embryos contained about 40 chromosomes, indicating that they were diploidized, as haploid parthenogenetic blastocysts have about 20 chromosomes. Results suggested that haploid-derived parthehogenetic cells in chimeric embryos diploidized spontaneously after the blastocyst stage. These cells were capable of differentiating into most cell types of embryonic tissues, but scarcely differentiated into extraembryonic tissues of day 9 embryos. The fate of haploid-derived parthenogenetic cells during postimplantational development was similar to that of diploid parthenogenetic cells that had been diploidized experimentally in the one-cell stage.