Soybean is an autogamous crop species, and the production of large numbers of hybrid seeds has been tedious. Nevertheless, hybrid seed is important to certain types of research. Insect pollination of male-sterile plants facilitates the production of hybrid soybean seed. A new method of hybrid soybean seed production, the Cosegregation Method, which was developed in this dissertation research, utilizes close genetic linkage between a male-sterility locus, Ms6, and a seedling marker locus, W1. This closely-linked marker system allows identification of male-sterile plants at the seedling stage and improves hybrid seed production through greater efficiency and higher yield of hybrid seed per male-sterile plant. The Cosegregation Method was used to produce testcross (hybrid) seed for genotypic evaluation of germplasm. Seven lines were evaluated with two testers. Vegetative heterosis was expected to precondition testcross plants to early lodging, pod abortion, and grain yield reduction. Near-isogenic tester lines containing alleles affecting stem growth (normal, semideterminate, and short) were used to try to reduce vegetative heterosis and increase reproductive heterosis. Significant midparent heterosis and inbreeding depression, but not high-parent heterosis, was observed for nearly all traits. Concurrent use of per se and F1 heterosis evaluation was considered most appropriate for germplasm evaluation. The choice of tester was important in determining parental value of germplasm. The semideterminate and short-stem types may be of value in commercial hybrid soybean lines, but were not useful to germplasm evaluation. The Cosegregation Method also can be used to facilitate S1 and half-sib (population or inbred line as tester) recurrent selection for population improvement of complex agronomic traits by improving evaluation through the elimination of male-sterile plants as seedlings, and by allowing greater control of parental contributions during intermating. Development and use of the Cosegregation Method will assist in the elucidation of the genetic control of complex traits, the identification of lines to improve these traits, the improvement of populations for these traits through the use of recurrent selection, and determination of the agronomic potential of commercial hybrid soybean.