Contraceptive knowledge and behaviour of mothers of large (five of more children) and small (under three children) families in four subcu;tures were compared with white Protestants. Four hundred and forty-nine mothers aged 35–45 years were studied from black, Cuban, Indian, Chicano, and white groups. With social class, knowledge of birth control, and degree of religiosity held constant, the best predictors of family size were the mother's desited family size (expressed as desired minus actual children) age at childbirth, and age at mariage. Data suggest that family size is not purely a function of birth control knoeledge but related to early marrage and pregnancy and in keeping with attitudes about an ideal family size. In general, factors related to size were stronger in the white group than in the subcultures, and in a few instances certain cultures were not consostent with others in overall trends.