THE complexity of the life cycle of malaria parasites and the lack of suitable strain characters which could be used as genetic markers have made genetic studies of these organisms difficult. We have now available, however, a number of strains of Plasmodium berghei isolated from wild rodents and mosquitoes in Africa, which can be differentiated from one another by starch-gel electrophoresis of certain enzymes1. We have also derived parasite lines resistant to the drug pyrimethamine from some of these strains, and such resistant lines remain stable even in the absence of the drug. By making crosses between lines which differ in both enzyme type and drug sensitivity, we have been able to obtain parasites exhibiting recombinant characters.