To address the question of how many distinct parasites are injected when a mosquito bites, we have characterized isolates resulting most probably from a single sporozoite inoculum. We describe the direct and immediate cloning on hepatocyte feeder layers of a Thai and an African Plasmodium falciparum primary isolate and the characterization of 67 independent clones by four techniques totaling nine different markers. This led to three main conclusions: (a) both the phenotypic and genotypic markers revealed an unexpectedly large degree of diversity within the clones from a single isolate; (b) the clones are nonetheless genetically related; and (c) a single mosquito inoculum would most likely be sufficient to generate considerable isolate complexity in the absence of repeated exposure. This diversity, which has been greatly underestimated in previous studies, does not bode well for the development of successful malaria control means.