Data for a group of research chemists are examined to test the hypothesis that marital fertility is associated with lower levels of research productivity and to assess the extent to which the fertility-productivity relationship varies by sex. The results show a negative relationship between fertility and productivity among the scientists; the relationships for the sexes are similar in direction and magnitude. We argue that previous studies of the impact of marital fertility on career attainments have failed to find the expected negative association because they have dealt with workers who cannot freely invest extra time and energy in work efforts and because they have used inadequate measures of attainment such as occupational mobility rather than more direct measures of job performance.