The recent outbreaks of pea aphid, Illinoia pisi Kalt., in southern California have followed mild winters with low rainfall, which allowed the survival of many viviparous females and their rapid multiplication with the advent of spring. The first growth of alfalfa in infested fields was destroyed, the second growth weakened, and weeds enabled to grow and mature in abundance as a result of the retardation of the alfalfa. The epidemics ended with the coming of hot weather, due to development of insect enemies, failure of food supply and shade, and direct killing effect of heat and low humidity. A number of artificial control methods were tested. Those of practical value were cultural practices for the purpose of eradicating winter alfalfa growth and attendant over wintering aphids in the fields, the use of burning machines, contact sprays or calcium cyanide for newly sprouted alfalfa, and early cutting. Those of no value were brush dragging, the Dudley aphidozer, and cultivation after active growth had commenced in the spring. Introduction of Hippodamia convergens lady-beetles was tried with negative though suggestive results.