This paper describes the fertility, mortality, and marriage patterns of the Hungarian Settlement, Louisiana, USA, immigrant ethnic population and relates these demographic processes to the evolutionary forces of natural selection, genetic drift, and gene flow. The results indicate that the maximum opportunity for natural selection decreased over time, and natural selection could have operated in the case of this population at only a very moderate level. The demographic characteristics of this population suggest that genetic drift may be important as an agent of microdifferentiation. Gene flow, however, appears to be the most important evolutionary force in this population. The process, based on the increasing incorporation of non-Hungarians into the gene pool, is causing the breakdown of this ethnic/genetic isolate.