Since a knowledge of the susceptibility of any species of Anopheles to infection with malaria parasites is of great importance in determining its part in the transmission of malaria, the experiments reported here were undertaken, and included the three most prevalent species of this genus occurring in the United States. As a result of these experiments Anopheles punctipennis is shown to be an efficient host of the organisms of tertian and estivo-autumnal malaria, Anopheles crucians of estivo-autumnal malaria, at least, and information has been obtained upon the relative susceptibility of these two species and Anopheles quadrimaculatus. The latter species has been known to be an efficient host since Thayer's experiments in 1900, and has been considered to be the principal species concerned in the transmission of malaria in the United States. With Anopheles punctipennis, developmental forms of the exogenous or sporogenic cycle of Plasmodium vivax were demonstrated in six (85 per cent) of the seven mosquitoes dissected, and the development of Plasmodium falciparum, in four (20 per cent) of twenty specimens. These four infections, however, occurred in a series of thirteen specimens fed on one person, so that the percentage was actually 33. With Anopheles crucians, oocysts or sporozoites or both oocysts and sporozoites of Plasmodium falciparum were found in nine (75 per cent) of the twelve specimens dissected. No tests were made with this species and Plasmodium vivax. Anopheles quadrimaculatus was employed as a control species in the experiments and became infected in the following ratio: eight (66 per cent) of twelve specimens with Plasmodium vivax, and three (15 per cent) of nineteen specimens with Plasmodium falciparum. In determining the relative susceptibility of the three species only those individuals which had fed upon the same gamete carriers are considered. The number of mosquitoes from which the percentages are computed is too small to make the results entirely conclusive, but the indications are that Anopheles punctipennis and Anopheles quadrimaculatus are equally susceptible to infection with Plasmodium vivax, 85 per cent of each species under the same conditions being positive. With Plasmodium falciparum,Anopheles crucians showed the highest percentage of infection (75 per cent), Anopheles punctipennis second (33 per cent), and Anopheles quadrimaculatus third (23 per cent).