Morphological studies of mouse embryos (F1hybrids between inbred strains of CBA females and CBA-T6T6 males) on the 9th, 10th, and 11th day of development revealed a characteristic disturbance in embryogenesis, inhibited development of the embryonic part of the egg-cylinder. This disturbance first appeared on the 9th day but did not affect the fetal membranes, which retained their normal appearance until the 11th day. All the egg-cylinders characterized by this anomaly were triploid. The presence of only one T6 marker chromosome showed that triploidy occurred as a result of digyny.