The occurrence of multiple-clone Plasmodium falciparum haploid blood-stage infections is a pre-requisite for cross-fertilization and genetic exchange at the diploid stage in the mosquito. Using monoclonal antibodies against 3 polymorphic blood-stage antigens, a method of two-colour differential immunofluorescence allowed the resolution of between 1 and 4 clones/isolate. A mean of 2 P. falciparum clones was detected in the blood of malaria patients in The Gambia. The mean number of clones/patient showed no correlation with age, parasitaemia, or disease severity. There was a slight difference in mean number between sample periods, probably reflecting temporal differences in transmission intensity. A statistical analysis of 2-locus genetic diversity of clones within isolates concludes that not all multiple-clone infections result from superinfection, but that some are due to single multiple-clone inoculations.