Sex-linked occupational differentiation has been seen as influenced by both the industrial structure of the economy and the sex composition of the labor force. Here, with a sample of 70 SMSAs, it was found (a) that the odds of men relative to women of joining professional and managerial occupations increased between 1960 and 1970, and (b) that this increase was dependent on the growth of tertiary industries and the greater number of women joining the cash economy. The observed effect of industrial shifts on sex-occupational differentiation, however, is argued to be a spurious consequence of the gender-composition of the work force. Specifically, the development of tertiary industries generates greater demand for female labor. Intensive recruitment of women to the labor force in turn increases occupational differentiation because females, in sex-typed labor markets, are likely to be channeled in disproportionate numbers away from upper-status occupations. The findings demonstrate that traditional modernization theory is unable to account for this. However, the results lend support to expectations derived from a labor market sex-segmentation approach.