When three lines (R1, R2 and R3) of the pea aphid ( Macro- siphum pisi (Harris)) were reared simultaneously for 9 days at 69° F. each on one susceptible (Perfection) and two resistant (Pride and Onward) varieties of peas, Pisum sativum L., the lines differed in mean weights and numbers of females produced. R1, R2 and R3 females averaged respectively 3.4, 2.6 and 2.0 mg. on Perfection and 2.4, 1.4 and 1.0 on Onward; the mean numbers of progeny per three females were 192, 134 and 101 on Perfection and 75,20 and 35 on Onward. Over activity in adults maintained on Pride and Onward was apparently associated with under-nourishment. However, nymphs born on the resistant varieties reached the adult stage without any sign of over activity.