Pratylenchus penetrans andP. crenatus were the dominant plantparasitic nematodes in potato roots and soil in Prince Edward Island. No nematodes were recovered from potato tubers. Forage legumes and grasses grown in rotation with potatoes also harbored large populations of root lesion nematodes. The amount of water moving through the stems of potato plants infected withP. penetrans in the greenhouse generally was less than in nematode-free plants;P. crenatus did not produce this effect. Telone-II, Telone-C-17, and Temik reduced the number of nematodes substantially in field plots. Yields of Superiors, Kennebecs, Sebagos and Netted Gems in nematicide-treated plots were not significantly greater than in untreated plots. There was a small but significant reduction in the specific gravity of tubers in the Telone-treated plots.