Results of egg transfer and of reciprocal crossing among breeds of sheep with a range of 1 week in mean gestation period indicate that the genotype of the fetus is the major determinant of the duration of gestation, over a range of litter sizes from 1 to 5 and a wide range of combinations of size of donor and recipient breeds. It is concluded that the fetus accounts for at least two-thirds of the genetic variation in gestation period in this species.