For both white and non-white births a definite inverse relationship between sex ratio at birth and the age of the mother and between sex ratio at birth and birth order when age of the mother and birth order are considered independently. For white births the sex ratios at birth vary inversely with birth order and to some extent this is also the case for non-white births. An apparently similar relationship of sex ratio with age of the mother is due largely, if not solely, to the fact that lower orders of birth are more predominant amongst younger mothers. Birth order has a definite effect on sex ratio at birth, regardless of age of the mother, whereas age of the mother appears to have relatively little effect on sex ratios at birth except insofar as there is correlation between age of the mother and birth order, the latter being the controlling factor.