The population structure of wild oats (Avena fatua) sampled in two prune orchards was described using Wright's model of a population having many largely isolated, small subdivisions. A high degree of genetic differentiation was observed among the individual colonies for lemma color, leaf sheath hairiness and isoenzymatic loci. Estimates of genotypic frequencies and population sizes over a two-year period suggested that random drift played an important role in the population changes toward a highly mosaic pattern of differentiation and local monomorphism in a substantial proportion of colonies. It was recognized, however, that without additional extensive field studies, the hypothesis of irregularly dispersed factors of multiniche selection could not be ruled out. Similar studies are briefly reviewed in order to outline the research needed on the issue of selection versus random drift as the primary force in local differentiation.