Malaria is a major human parasitic disease caused by four species of Plasmodium protozoa. Plasmodium vivax, the most widespread, affects millions of people across Africa, Asia, the Middle East, and Central and South America. We have studied the genetic variability of 13 microsatellite loci in 108 samples from 8 localities in Asia, Africa, South America, and New Guinea. Only one locus is polymorphic; nine are completely monomorphic, and the remaining three are monomorphic in all but one or two populations, which have a rare second allele. In contrast, Plasmodium falciparum displays extensive microsatellite polymorphism within and among populations. We further have analyzed, in 96 samples from the same 8 localities, 8 tandem repeats (TRs) located on a 100-kb contiguous chromosome segment described as highly polymorphic. Each locus exhibits 2-10 alleles in the whole sample but little intrapopulation polymorphism (1-5 alleles with a prevailing allele in most cases). Eight microsatellite loci monomorphic in P. vivax are polymorphic in three of five Plasmodium species related to P. vivax (two to seven individuals sampled). Plasmodium simium, a parasite of New World monkeys, is genetically indistinguishable from P. vivax. At 13 microsatellite loci and at 7 of the 8 TRs, both species share the same (or most common) allele. Scarce microsatellite polymorphism may reflect selective sweeps or population bottlenecks in recent evolutionary history of P. vivax; the differential variability of the TRs may reflect selective processes acting on particular regions of the genome. We infer that the world expansion of P. vivax as a human parasite occurred recently, perhaps <10,000 years ago.