Glyphosate, applied early or later or twice to genetically modified glyphosate-tolerant sugar beet, gave excellent control of planted 'volunteer' potatoes growing within the crop compared to conventional herbicide programmes with or without clopyralid. In three out of four trials, this resulted in significant reductions in the numbers of eggs and cysts of potato cyst nematodes (Globodera rostochiensis and G: pallida) where infestations were moderate (23-89 eggs g(-1) soil). In the fourth trial, which had very high initial populations (130 eggs g(-1) soil), none of the herbicide treatments had any significant effect on numbers of nematode eggs or cysts. This was probably due to competition for feeding sites, and the early death of the potatoes in all treatments caused by feeding damage by the nematodes and infection by blight, which prevented the nematodes from completing their life cycle. Glyphosate also significantly reduced the number and size of daughter tubers produced, thus helping to prevent a further volunteer problem in the next crop in the rotation. This was achieved by one or two applications of one chemical compared to 2-5 applications of cocktails of conventional herbicides.