Broadening the genetic base of upland cotton (Gossypium hirsutum L.) is essential for continuous genetic improvement of yield and fiber quality through breeding. The objectives of this study were to evaluate a species polycross (SP) population for phenotypic and genotypic variations in yield and fiber quality, investigate morphological variations among the SP lines, and analyze the interrelationships among yield, yield components, and fiber properties. A population was developed by crossing cultivars and strains of G. hirsutum with the other tetraploid species in the genus. This SP population underwent 11 yr of random mating by natural pollination in an isolated environment with high bee activity followed by 12 yr of selfing. The experiment was conducted at two field locations with two replicates each. Two hundred and sixty lines of the SP population were evaluated with five commercial cultivars. Genotypic variation was significant (P < 0.01) for all characters of yield and fiber quality. Although a highly significant (P < 0.01) interaction between genotype and location was identified for fiber strength and most yield parameters, the interaction components were small relative to the genotypic components. There was large variation among the SP lines for nectary size, gland content in leaves, pubescence, leaf cut depth, plant height, leaf area, leaf length, and node number of the first fruiting branch. Span length (50%) contributed more variation to fiber strength than span length (2.5%) in the SP population. Lint yield was negatively correlated with short fiber content. It is concluded that the SP population is a useful germplasm for genetic improvement of lint yield and fiber quality.