Twins totaling 569 pairs were studied in relation to type of twin placentation. Separate diamniotic-dichorionic placentas were rarely associated with monozygosity. This and the fact that monochorionic placentas are all monozygotic make examination of twin placentas extremely useful in forecasting zygosity. Relatively heavy infants as well as twins with large intrapair birth-weight differences were common in the separate diamniotic-dichorionic group, suggesting independent intrauterine growth of co-twins. The converse was true in twins with fused diamniotic-dichorionic placentas, who also had the lowest death rates. Perinatal death rate Was the same in whites (14.7%) and Negroes (14.3%), highest in male-male twin pairs, but lowest in male- female pairs in Negroes and female-female pairs in whites. Compared to singleton deaths, the frequency of congenital malformations was not higher in twin deaths, but the types of malformations found in monochorionic deaths were often multiple and lethal.