During the spring of 1939 a severe outbreak of the green bug (Toxoptera graminum) occurred in ne. Oklahoma and caused heavy loss in small grain crops. Winter barley was more severely damaged than winter wheat and a much greater % of fields was infested. Greatest damage was inflicted on spring oats. There is evidence of comparative resistance of some oats vars. to green bug damage. Fall-planted barley, oats and wheat were much more severely damaged following grain sorghums (darso) in a rotation than following soybeans, maize, or wheat. In all fields studied, the parasite Lysiphlebus testaceipes did not occur in sufficient abundance to check the green bug until after the infestation had declined to a low point or until the grains had already been damaged beyond recovery. The most important predator was Hippodamia convergens, the larvae of which were abundant at the peak of the green-bug infestation.