We exploited the high resolution capacity of polymerase chain reaction (PCR)-coupled single-strand conformation polymorphism (SSCP) to screen for sequence variation in the second internal transcribed spacer (ITS-2) of ribosomal DNA (rDNA) among 77 individuals representing Oesophagostumum bifurcum from human or Mona monkey hosts from Africa. SSCP analysis revealed distinct profiles among some of the individuals, and sequence analysis of representative samples defined different ITS-2 sequence types attributable to polymorphism at particular nucleotide positions. However, there was no unequivocal sequence difference between O. bifurcum individuals from humans and that from monkeys. This provided some support for the hypothesis that the parasite from the two hosts represents a single species and that the sequence microheterogeneity detected in the ITS-2 rDNA region represents population variation, although the findings were insufficient to reject the proposal that the parasite represents different species. Overall, the results showed the usefulness of the SSCP-sequencing approach for studying the genetic variation in O. bifurcum populations and indicated its potential to study macromolecular evolution and elucidate population differentiation at the molecular level.