Although we regard Anopheles quadrimaculatus as a very good host for certain strains, at least, of Plasmodium vivax, the circumstance that in most lots of this species which have been given an infecting feed, there will commonly be observed a varying proportion of individuals which did not become infected, suggests that some mosquitoes are definitely refractory to infection. The infection acquired by the susceptible members of a lot exhibits qualitative and quantitative characteristics. The former, as expressed by us, is represented by the number of infected specimens discovered in a lot, the latter by the number of cysts developing on the stomachs of those which become infected. It has been shown (1) with high probability, that the quantitative infection will vary directly with the gametocyte density in the infectious patient, and that although the qualitative infection may likewise be influenced by the same factor, there is definite suggestion that chance variations in the proportions of refractory and susceptible mosquitoes in a lot also influence the qualitative results.