This paper summarizes the result of investigations carried on by the writer in the last two years on the parasites of the aphis feeding Syrphidae of Southern California. The writer found that there are several species of hymenopterous parasites constituting the most important natural checks of these predacious flies. In certain seasons these parasites reproduced very rapidly at the expense of their hosts to the extent that the efficient work of these flies was greatly hampered, and consequently the aphids reproduced freely, causing serious damage to the infested alfalfa fields. No one species of these parasites can be regarded as exercising the most effective check on the flies, but there exists a sequence of several hymenopterous parasites persistently attacking the larval and pupal stages. Occasionally, one or the other of these enemies appear in such force as to be a serious menace to the syrphids. The seasons most favorable to parasite infestation are the spring and summer with the exception of the ichneumon, Syrphoclonus spp., whose percentage of parasitism for the various parasites has been as high as forty-five, over eighty per cent of which were the result of attacks by the ichneumon species during the fall and spring. In the summer, however, the Diplazon laetatorius and the chalcid parasites rank first in importance. Life history studies as well as the methods by which the parasitic larvae feed, respire, dispose of waste material will be briefly discussed. Important problems having economic or biologic bearings are dealt with in the text.