Monoclonal antibodies against variant epitopes of a highly polymorphic protein (PV200) in schizonts of Plasmodium vivax have been used to analyze the variety of genetically distinct populations of parasites present in the peripheral blood of individual P. vivax infections in Sri Lanka. In 9 out of 10 isolates of freshly drawn P. vivax infected blood from different individuals, parasites of only 1 PV200 serotype was found within each individual infection, even though parasites were serotypically distinct between individuals. In 1 isolate parasite population, 3 distinct PV200 serotypes were identified. Thus, most P. vivax infections appeared to consist of a single genetically homogeneous population of parasites within the detection limits of the technique. The prevalence of P. vivax infections in an area of malaria transmission in southern Sri Lanka and the densities of oocysts in mosquitoes fed on P. vivax infected individuals indicated that parasite populations would be transmitted many times before encountering parasites of other origins, and that individual populations would tend to reduce to genetic homogeneity during transmission. These expectations are consistent with the high proportion of genetically homogeneous P. vivax isolates observed.