A tetraploid single-cyst isolate of Heterodera glycines from a field population from Indiana has been propagated in the greenhouse on Lee soybeans since its discovery, in 1973. The tetraploid isolate has n = 18 chromosomes, compared with n = 9 of the diploid H. glycines; it has larger cysts and larvae, but shows the same level of parasitism and host range as the diploid population from which it apparently evolved. Association of chromosomes is irregular at metaphase I, with quadrivalents, trivalents, and univalents often observed in addition to the bivalents. The second maturation division is usually normal. About 80% of the mature oocytes (just before fertilization) have n = 18, and the other 20% have n = 17 or 19. Reproduction of the tetraploid isolate is exclusively by cross-fertilization. The discovery of such a tetraploid provides an experimental tool for the study of polyploidy in nematodes. Many amphimictic plant-parasitic nematodes are suspected of representing polyploids.