Genetic markers in the mitochondrial genome have proven useful for population genetic studies because of their maternal inheritance and relatively high evolutionary rates. In this study, we exploited the high resolution capacity of PCR-coupled single-strand conformation polymorphism (SSCP) to screen for sequence variation in part of the cytochrome c oxidase subunit 1 gene (p cox 1) among individuals of the parasitic nematode, Oesophagostomum bifurcum from human or Mona monkey hosts from Africa. SSCP analysis revealed distinct profiles among some of the individuals, and subsequent sequence analysis of representative samples defined 10 different haplotypes. For comparative purposes, the p cox 1 sequences for representatives of four other species of Oesophagostomum from livestock were included. While there were high levels (11.5-13.7%) of sequence difference among the latter species, there was no fixed nucleotide difference between O. bifurcum individuals from humans and those from monkeys. The data support the proposal that O. bifurcum from the two primate hosts represents a single species and that the haplotypic variability in p cox 1 represents population variation. The results reinforce the usefulness of the SSCP-sequencing approach for studying genetic variation in nematode populations using mitochondrial markers.