A mutation scanning approach was employed to investigate the population genetic structure of the bovine lungworm, Dictyocaulus viviparus (Nematoda: Trichostrongyloidea), in southern Sweden. A total of 252 individual nematodes were collected from cattle representing 17 farms. A portion of the mitochondrial cytochrome c oxidase subunit 1 gene (pcox1) was amplified from genomic DNA isolated from individual lung-worms by the polymerase chain reaction (PCR), and then subjected to single-strand conformation polymorphism (SSCP). Samples with distinct SSCP profiles were then sequenced. In total, 12 distinct pcox1 haplotypes (393 bp) were defined for the 252 individuals, and pairwise sequence differences among the haplotypes ranged from 0.3-2.3%. Average haplotype diversity and nucleotide diversity values were 0.16 and 0.002, respectively. There was no particular correlation between pcox1 haplotypes and their geographical origin. The "overall fixation" indices F-ST and N-ST were calculated to be 0.77 and 0.65, respectively. The results of this study revealed that both the mitochondrial DNA sequence diversity within populations and the gene flow among populations of D. viviparus were low. This is similar to findings for some parasitic nematodes of plants and insects, but distinctly different from gastrointestinal trichostrongyloid nematodes of domesticated ruminants considered to have relatively high levels of genetic diversity and gene flow. Such differences were interpreted to relate mainly to differences in host movement as well as parasite biology, population sizes and transmission patterns, and should therefore be of epidemiological relevance.*