As a follow-up on the studies by Dorn and Beale, this paper examines differences between Iowa counties with natural decrease and those with natural increase and analyzes the part that migration and fertility played in bringing about an excess of deaths over births in Iowa counties. The county groups are distinctly different in demographic and socioeconomic characteristics. Out-migration as a mode of response adopted by the rural population in Iowa is by far the most dominant factor leading to natural decrease. Sustained net out-migration is more likely to touch off natural decrease in counties of comparatively low fertility than in those with higher fertility. Low fertility is, therefore, a contributing factor to the imbalance between births and deaths, but the amount of influence of fertility adjustment over the fertility differentials among county groups cannot be ascertained in this study because of the correlation between fertility and Catholic Church membership in counties. The data of this study were obtained from the population censuses and vital statistics.