Technical Abstract: The effects of environmental conditions on population trends of plant parasitic nematodes were studied in experimental plots of five wheatgrasses in the western Utah desert. Soil water and temperature affected the population trends of the ectoparasitic stunt nematode, Tylenchorhynchus acutoides, the ectoparasitic dagger nematode, Xiphinema americanum, and the migratory endoparasitic lesion nematode, Pratylenchus neglectus, on Fairway crested wheatgrass, Agropyron cristatum; 'Hycrest' crested wheatgrass, A. cristatum X A. desertorum, 'Rosana' western wheatgrass, Pascopyrum smithii; 'Oahe' intermediate wheatgrass, Thinopyrum intermedium; and RS-1 hybrid (Elytrigia repens X Pseudoroegneria spicata, in a three-year field study (1984-86). The largest soil populations were collected in 1984 under good plant growth conditions. A reduction in nematode populations occurred in 1985 and 1986 apparently because of low soil-water conditions. There was a positive relationship between high soil-water and maximum population density peaks of T. acutoides in the spring and fall of 1984, and low soil water and minimal population densities of the nematode in 1985 and 1986. Pratylenchus neglectus populations were affected by soil water, although to a lesser degree than the ectoparasitic nematodes. Nematode population densities were significantly lower in 1985 and 1986 than in 1984. Higher nematode populations were found in the spring on the earlier-maturing cultivars Hycrest and Fairway. Maximum populations occurred later in the season at a lower soil depth.