Abstract The phenomenon of hybrid vigor is well known from the crossing of both plants and animals. The first generation hybrids resulting from the crossing of two pure breeds frequently exhibit vigor superior to either of the crossed pure breeds. The high yielding qualities of first generation hybrids in corn have been demonstrated by Jones2 and others. Practical breeders of swine have been convinced of the exceptional vigor of individuals resulting from the first generation of crosses between pure breeds. In poultry superior vigor of hybrids has been utilized commercially, in the production of both market poultry and eggs in England. Although it has been generally accepted that first generation hybrids exhibit exceptional vigor, to the writer’s knowledge, no data have been presented accurately measuring the stimulation resulting from hybridization of poultry. In the following pages data are presented comparing the vigor of hybrids with that of the two. . .