Five weedy biotypes of proso millet (Panicum miliaceum L.) were grown under conditions of both intra- and inter-biotype competition. These biotypes represent the range of weediness from crop-like, large, white-seeded types with dense, drooping panicles to the most weed-like strain, which produces small, dormant, black seeds and has open, shattering panicles. Under noncompetitive conditions, the five biotypes of proso millet (WHITE, GOLDEN, ORANGE, CROWN and BLACK) differ strikingly in plant size, fresh weight distribution patterns, flowering time, and seed size. All biotypes showed decreased biomass per plant and delayed flowering time with increasing density when grown in monoculture. Variation in total biomass among individuals in each biotype was reduced at increasing densities resulting in positively skewed frequency distributions. The crop-like biotypes (WHITE and GOLDEN) demonstrated the greatest intrabiotype competitive effects, i.e., were the least tolerant of increased density in monoculture showing decreased survival and reduced seed weight. When grown in mixed stands, the same two biotypes were found to have the greater competitive ability than the other biotypes in terms of both survival and total standing crop biomass. The contributions of the BLACK, CROWN, and ORANGE biotypes to total biomass in mixed stands were very small. Factors affecting the successful establishment of field infestations of the crop-like and weedy biotypes are discussed.