Meiotic recombination generates allelic diversity in the Plasmodium falciparum merozoite surface protein 1 (msp1) gene. In this study, we monitored recombination-based diversity of msp1 in Guadalcanal, the Solomon Islands, where malaria transmission is high. We identified 5' recombinant types, 3' sequence types, and msp1 haplotypes (unique associations of 5' recombinant types and 3' sequence types), and compared them with those from areas of low transmission in Thailand and Vanuatu. The mean number of 5' recombinant types per person (multiplicity) was lower in Guadalcanal than in Thailand. Guadalcanal populations had 6-8 msp1 haplotypes; the numbers are comparable to Vanuatu but much lower than in Thailand. There were marked geographic differences in distribution of msp1 haplo-types. Linkage disequilibrium in msp1 was stronger in Guadalcanal than in Thailand, suggesting limited recombination events in the Solomon Islands. We suggest that the frequency of recombination events in msp1 is determined not only by transmission intensity but by the number of msp1 alleles prevalent and multiplicity of infections.