Evidence is presented that genetic variability affecting duration of seed dormancy and seed viability exists in field populations of Avena fatua L. and that this variability is an important factor in the persistence of this weed species in allowing populations to adapt to particular cultural weed-control practices. Duration of seed dormancy is controlled by several or many genes, the effects of which are manifested in at least two physiologically distinct physiological blocks. Expression of genotypes which confer long-term seed dormancy is highly sensitive to temperature and drought stress experienced by maternal plants during seed development. Naturally inbreeding families characterized by long-term dormancy also exhibit rigorous hormonal control of the synthesis of at least one hydrolase in endosperm (α-amylase). This association of physiological traits apparently is not due to pleiotropy but rather to coadaptive multilocus organization.