Nine hundred game-farm pheasants (Phasianus colchicus), from California stock, and 814 wild-trapped pheasants, from east-central Illinois, were released during the winters of 1959-60 and 1960-61, on an area near Neoga, located about 20 miles south of the range occupied by self-maintaining populations in Illinois. This experiment, termed the gene pool concept, represented initial efforts to test ... [Show full abstract] the ability of crosses among different species, subspecies, and strains of pheasants of the genus Phasianus to produce self-maintaining populations south of the contiguous range now occupied by pheasants in Illinois. Wild Illinois pheasants survived in significantly greater numbers than California birds. Survival of the sex and age groups of each strain was similar. Reproductive success of the wild Illinois birds and of their progeny, in 1960 and 1961, was similar to that of pheasants in self-maintaining populations; California birds, for the most part, failed to reproduce. The rate of death among adult hens during late summer was probably higher at Neoga than within the established pheasant range. Survival of pheasant hens from October to the following May averaged 28 percent for the 2 years, 1960-61 and 1961-62, and was apparently insufficient to allow establishment and continuation of the population. Factors preventing the southward extension of self-maintaining pheasant populations in Illinois are concerned more with survival than with reproduction.