Differences in resistance of varieties of peas to the pea aphid, Acyrthosiphon pisum (Harr.), have long ago been shown in the field and by controlled rearings under glass, Russell and Morrison (1924, p. 63) apparently being the First to observe differences. Early investigations at Wisconsin by Searb (1932) showed that pea varieties differ in the number of aphids born upon them. Maltais (1937) corraborated these early findings and reported (1950) the development of a pure-line strain of aphid-resistant pea. The influence of resistance in peas upoil aphid development, reproduction, and longevity as investigated by Harrington (1941) with individual plants under controlled conditions in the greenhouse. His results confirmed the inability of the insect to develop and reproduce as fast on a resistant plant as on a susceptible one. Tt has been postulated that the composition of the available food in the host plant plays possibly the most important role in determining the resistance to aphid attack (Evans, 1938; Painter, 1941). Maltais (1948), Auclair and Maltais (1950), and Maltais (1951) gave preliminary reports on the probable factors in varietal resistance in peas.