Allelic diversity in the merozoite surface protein-1 (MSP-1) of Plasmodium falciparum, a major malaria vaccine candidate, was examined in clinical isolates from holoendemic northern Tanzania.

The variable blocks 2,4a, 4b, 6, and 10 of the MSP-1 gene were typed by allelic type-specific polymerase chain reaction.

Twenty-four possible MSP-1 gene types were defined as unique combinations of allelic types detected in each variable block.

Thirteen gene types were identified, and 187 P. falciparum populations were fully typed among 79 isolates.

In contrast with recent findings in Vietnam, we were unable to detect nonrandom associations between allelic types in the typed variable blocks.

Most patients (60%) harbored more than I genetically distinct parasite population (average : 2.37 populations per isolate) and, in I patient, 6 different versions of this single-copy gene were found.

Statistical analysis suggests that parasites carrying different MSP-1 gene types are not independently distributed in the host population.

The epidemiological consequences of these findings are discussed.