During 1941 and 1942, aphid (Macrosiphum pisi) collections were received from 12 widely separated pea-growing areas of the U. S. and tested under uniform greenhouse conditions at Mil-pitas, Calif., to determine whether > 1 biol. race of the insect is present in the U. S. Expts. involving comparative feeding injury, body size, and rates of reproduction indicate that the pea aphid species is a species-complex, composed of many races. All data obtained were subjected to appropriate statistical analyses, and on this basis the 31 aphid lines tested were tentatively assigned to 5 separate biol. races.