Twenty six populations of potato cyst-nematodes from Colombia, Ecuador, Peru and Bolivia were examined and identified as Heterodera rostochiensis Woll. or H. pallida Stone. North of 13.6°S only H. pallida was found, but south of this latitude most populations examined were H. rosto- Mensis or a mixture of the two species. Possible reasons for the distribution of the two species being dependent on latitude are discussed, particularly the influence of daylength. © 1975, Brill. All rights reserved.