Chromosomes were studied in mouse embryos and foetuses recovered after superovulation. The spontaneous incidence of chromosomal anomalies was 2·6%; a delay of 7 to 13 hr before fertilization produced 3·9% anomalies. The most striking feature was a nine-fold increase in the incidence of triploidy after delayed fertilization. The chromosomal anomalies included isochromosomes and minute chromosomes. Some anomalous individuals survive implantation and the fate of such foetuses is discussed.