Isolates of Plasmodium falciparum from patients in a Sudanese village exhibiting RI resistance to chloroquine have been typed for allelic variants of 2 merozoite surface antigens, MSP1 and MSP2. Blood forms were taken from each patient before chloroquine was administered, and after parasites had reappeared following treatment. Each patient was found to be infected with genetically different parasites. However, in each patient the parasites of the recrudescent infections possessed the same alleles of each gene as those of the primary infection. The results show that the parasites which reappeared after chloroquine were a genuine recrudescence of the primary forms, and not derived from a new infection.