Tolerance and resistance to P. thornei of seven CIMMYT-derived cultivars of wheat (Triticum aestivum), selected on soil naturally infested by Pratylenchus thornei and released from 1960-1997, were evaluated under optimum and limited irrigation conditions in northwestern Mexico. Replicated trials were grown in a naturally infested site in two seasons. Differences in yield, agronomic traits and nematode numbers were recorded in paired field plots, untreated or fumigated with dazomet to control nematodes. With optimum irrigation the nematode did not affect wheat yield but, with limited irrigation where plants were under water-stress, yield loss of all cultivars was comparable to that of the intolerant, susceptible control cv. Warigal (29%). There was a significant negative linear relationship (Y = –3.69 x + 3117) between initial nematode density (x) and grain yield (Y) under limited irrigation. None of these cultivars was resistant and post-harvest P. thornei populations had increased from three to 15 times the initial estimates. There was no relationship between date of release of cultivars and their responses to the nematode. There is a need for sources of resistance to be identified and incorporated into the germplasm.