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The second internal transcribed spacer (ITS2)* of the ribosomal DNA of Dictyocaulus viviparus from cattle, D eckerti from fallow deer. D filaria from sheep and D arnfieldi from donkeys has been sequenced to investigate the genetic relationships among lungworm species, especially between D viviparus and D eckerti, because the latter is not generally accepted as a separate species. The length of the ITS2 varied between 403 and 481 bases, and its GC content ranged from 25 to 33 per cent. Intraspecific variations in D viviparus (0 to 1.5 per cent) and D eckerti (0.6 to 3.3 per cent) were slight; sequence homology between the species ranged from 50.3 to 76.7 per cent. Some sequence differences occurred at restriction sites of endonucleases leading to characteristic restriction fragment length patterns. The interspecific differences between D viviparus and D eckerti far exceeded the intraspecific variation, thus providing additional evidence that the two species are genetically distinct.