Seeds of five yellow foxtail [Setaria lutescens (Weigel) Hubb.] biotypes, collected from Connecticut, Iowa, Massachusetts, Pennsylvania and California, were tested for dormancy and germination characteristics. Seeds of the California biotypes, while dormant at time of dispersal, germinated readily following a 4-month after-ripening period in dry storage. California biotype seeds did not require stratification. The eastern biotypes also showed high initial dormancy. Prolonged dry storage (16 months) or stratification overcame this dormancy to varying degrees. All five biotypes germinated over the range of 15 to 35 C, but the California biotype exhibited much higher germination at 35 C than any eastern biotype. The California biotype germinated more rapidly, by 1 to 3 days, at any given temperature than the eastern biotypes. These differences in dormancy and germination requirements between the five biotypes are considered as evidence to support a hypothesis of genetically controlled, physiological differences between geographically separated biotypes.