This paper examines birth seasonality in a rural U.S. county over the period 1911–1979. Data were taken from the complete series of birth certificates for the county population during this period. Birth seasonality was expected to reflect seasonal variation in agricultural workload earlier in the century (pre-1940) but to disappear as this population became more integrated into the wage labor market from 1940 to 1979. The pattern of birth seasonality in each decade from the 1910s to the 1970s by week, month, and season was analyzed with descriptive and analytic techniques. However, no birth seasonality was found for any decade during this century. The absence of a seasonal fertility pattern remained when all births were examined, and when such factors as ethnicity (only births to white mothers), occupation (only births to women married to farmers), and stillbirths were controlled singly and in combination. The implications of the lack of birth seasonality in this population are discussed in terms of the understanding of the causes of birth seasonality and the place of birth seasonality in human adaptation.