A strain of Plasmodium vivax from El Salvador was shown to be more infectious to the Apastepeque strain of Anopheles albimanus from El Salvador than was a strain of P. vivax from Colombia, S. A. The Apastepeque mosquito strain was also more susceptible to the Savadoran strain of P. vivax than were 2 other strains of this mosquito from El Salvador. A strain of A. albimanus from Panama was more susceptible to both of the parasite than were any of the Salvadoran strains of A. albimanus.