Plasmodium falciparum isolates were obtained from 17 pairs of Gambian children, each pair living in the same house and presenting with malaria at the same time. Frequencies of allelic serotypes of 3 polymorphic blood stage proteins (MSP1, MSP2, and Exp-1) were previously determined from a large number of isolates from patients in the local area, and the probability of a random pair of isolates containing an identical genotype was calculated to be less than 0.01. However, 3 of 8 household pairs in one year, and 6 of 9 in the next year, contained identical P. falciparum genotypes, a much higher frequency than expected randomly (P less than 0.00005, for each year). This finding is discussed in terms of the probable contribution of single mosquitoes infecting more than one person.