The distribution and frequency of occurrence of potato cyst nematodes, Globodera rostochiensis and G. pallida, were examined in Northern Ireland. Globodera rostochiensis (pathotype Rol) was the most abundant species, while Pal and Pa3 pathotype reactions could be discerned within G. pallida. Unlike other parts of the UK, species proportions appear not to have changed over the last 20 years, probably as a result of the legislative control currently practised combined with the limited use of Rol-resistant cultivars within the country. Differences in the geographical distributions of the three pathotypes were observed, and these are discussed in relation to original potato cyst nematode introductions and subsequent agricultural practices.