The Santa Lucia strain of P. falciparum was isolated from El Salvador, Central America, and established in A. trivirgatus monkeys. Transmission from monkey to monkey via the bites of infected Anopheles freeborni, A. maculatus and A. albimanus mosquitoes was obtained in 20 of 27 attempts. Prepatent periods in the monkeys ranged from 17-46 days with a mean of 24.3 days. Parasitemias and mortality were higher following sporozoite inoculation into animals previously infected with P. vivax than in those with no previous malaria experience. Monkeys previously infected with P. vivax and P. cynomolgi had lower maximum parasitemias than those previously infected with P. vivax only.