Technical Abstract: Meloidogyne chitwoodi Golden et al. is a serious pest of potato (Solanum tuberosum L.), and widespread in the Pacific Northwestern States of USA. M. chitwoodi is currently reported to consist of two host races and one pathotype (2, 3), which are not distinguished morphologically. Host race 1 reproduces on “Chantenay” carrot, but not on “Thor” alfalfa, and host race 2 reproduces on alfalfa, but not on carrot. Both races fail to reproduce on roots of S. bulbocastanum, a wild potato species used as source of resistance in our breeding program (1). The resistance to race 1 in S. bulbocastanum is attributed to Rmc1(blb) gene. Pathotype 1 of race 2 breaks resistance and reproduces on S. bulbocastanum (2). We have tested resistant breeding lines repeatedly in Prosser, WA field plots infested with MC race 1, and harvested tubers free from M. chitwoodi damage. In 2004, however, tubers of some resistant lines were damaged by the M. chitwoodi population that did not cause damage in the past. Populations of M. chitwoodi were established on tomato by adding peels obtained from the infected tubers of resistant lines. Like the MC race 1, new populations reproduced on “Chantenay” carrot (Reproductive Factor, final no. of eggs ¸ initial inoculum, 5000, > 1) but failed to reproduce on “Thor” alfalfa (RF < 0.1). Unlike MC race 1, they reproduced on roots of all breeding lines that carried Rmc1(blb) gene (RF > 1) These results suggest that the selected population of M. chitwoodi in the Prosser site is a pathotype 1 of MC race 1.