Reliable species determination of gastrointestinal nematode parasites from sheep is usually carried out on third-stage larvae (L3) by visual differentiation. The culturing of L3 takes about 1 week so a reliable method that could use eggs for the determination would accelerate the procedure. We developed a polymerase chain reaction (PCR), using very small amounts of parasite DNA and an oligonucleotide set according to parts of the DNA sequence of a beta-tubulin gene from Haemonchus contortus, that can discriminate H. contortus DNA and Trichostrongylus colubriformis DNA from each other and from several other sheep nematode parasites. This method can be used for every stage of the parasite, including eggs. For the first time a PCR method is reported to discriminate between sheep nematode parasites.