Abstract The hypothesis that the relationship between socio-economic status and fertility is changing from a negative one to a positive one as urbanization increases is examined. The Growth of American Families Study, conducted in 1955, provides data which are analyzed for two groups of people living in non-farm areas: indigenous non-farm couples and farm migrants. The authors conclude that when socio-economic status is measured by husband's income, the negative association disappears completely for the indigenous nonfarm couples and becomes slightly positive. When status is measured by wife's education, however, the pattern formed is more complex. There appears to be evidence of a transitional period in which the relationship shifts from negative in the marriages of longer duration to positive in the more recent ones.