Since it is recognised that a female rodent will litter best if left alone, it is rare for breeders to see the placenta, which is usually devoured by the mother immediately after the birth of the young. Work upon the milk factor for mammary carcinoma in mice has necessitated frequent examination of mice about to litter in order to exchange litters as soon after birth as possible, and thus a number of placentas have been seen when the mouse was caught in the act of littering. Nevertheless these represent only a small proportion compared with the number of mice born during this work. It is of some interest that two cases of placental fusion have been noted among this small series, which suggests that this condition may be fairly common among mice. A rough estimate of the frequency in this series is about 1 per cent. No reports of similar cases have been found in the literature, though a note by Merton (1938) that two embryos had been “implanted in the same place” may refer to a similar case. Because of the implications of this finding in relation to sexual and genetical differentiation in the mouse, a description of this material seems justified.