Broadening the genetic base of cotton, Gossypium hirsutum L., is essential for continuous improvement of yield and fiber quality through breeding. A germplasm population (SP) was developed by crossing cultivars in G. hirsutum with their relatives from wild tetraploid species of the genus. The SP population was initiated at North Carolina State University by P.A. Miller in 1966, and underwent 10 years of mixed selfing and random mating in an isolated environment with high bee activity followed by 12 years of selfing in an environment with predominantly selfing. Inbreeding is expected to be high within the lines in this population. The objectives of this experiment were to evaluate the germplasm for phenotypic and genotypic variation in yield, yield components, fiber quality, and morphological characters. The experiment was conducted at Stoneville, Mississippi at two locations in 2005. Two hundred and sixty SP lines were evaluated with five cultivars of G. hirsutum. Highly significant genotypic effects were identified for all characters analyzed. Wide ranges of mean values among SP lines were identified for yield, yield components and fiber properties.  Broad-sense heritability was estimated to be 0.31, 0.36, 0.34, 0.85 and 0.59 for fiber strength, span length (50%), span length (2.5%), elongation, and short fiber content. There was tremendous variation among SP lines for nectary size, gland content in leaf, pubescence, leaf cut depth, and other morphological characters related to canopy structure and earliness.  The results provide evidence that this SP population is a useful germplasm resource for genetic improvement of lint yield and fiber quality.