Seed dormancy, a major adaptive trait in plants, facilitates the survival of weeds and provides for resistance to preharvest sprouting (PHS) in cereal crops. Seventeen weedy strains and 24 cultivars of rice (Oryza sativa L.) were evaluated for germinability to screen for donors of dormancy genes. Extremely dormant genotypes were identified from the weedy strains. These genotypes displayed hull-and pericarp/testa-imposed dormancy. Three dormant weedy strains, LD, TKN12-2, and SS18-2, were crossed and backcrossed with the nondormant breeding line EM93-1 to determine the relationship between dormancy and the shattering, awn, hull color, and pericarp/ testa color characteristics. All these characteristics interrelated to the covering-imposed dormancy; the weedy forms of the characteristics significantly reduced germination in the BC 1F1 populations. Moreover, multiple linear regression analyses revealed significant effects of interaction between the characteristics on dormancy in the populations. The interrelation and interaction reflect the importance of combined effects of dormancy and other weedy characteristics in the adaptation of weedy populations to agroecosystems, and suggest that domestication and breeding activities have eliminated dormancy alleles at loci near the genes for shattering and the morphological characteristics from improved cultivars.