Stable RNAs have regions of primary sequence that are nearly identical in every member of the Plasmodium genus and not found in the host or in other common pathogens. Several "genus-conserved" sequences, which flank hypervariable regions, were identified within the small subunit ribosomal RNA of Plasmodium species. Primers based on these conserved sequences permit amplification of species- or possibly even strain-specific sequences from samples of unknown composition. As an example of this approach, sequences from the four human malaria species were successfully recovered from Giemsa-stained blood smears, including two different sequences for Plasmodium ovale (of 91.5% similarity). This type of information is useful for epidemiological and phylogenetic analysis of any malaria species. We show that amplification of rRNA-derived sequences behaves in a competitive fashion during the cycles of polymerase amplification and therefore target sequences from Plasmodium species are amplified in proportion to their abundance in the sample. There are several implications of this finding. (1) The proportion of different products resulting from amplification from samples with mixed infections is closely related to the proportion of infecting species. (2) Direct quantitation of parasite nucleic acids within a sample can be derived when known amounts of competitor RNA are added to the RT/PCR reaction. (3) Amplification of rRNA sequences, using genus-specific primers, allows one to monitor the development of the parasite in the mosquito.