In California, the expanding pistachio crop has been considered free of serious nematode diseases based on surveys in the 1980s with nematode- resistant rootstocks. During the 1980s, Verticillium wilt started to occur more frequently in pistachio orchards on susceptible rootstocks. This led to the selection and shift to Verticillium wilt-resistant rootstocks of crosses of P. atlantica and P. integerrima, , propagated as seedling or clonal 'UCB1'. In prior research, seedling populations of UCB1 had exhibited differences in nematode susceptibility, leading to the here described testing of commercial UCB1 clones for susceptibility to Pratylenchus vulnus and Meloidogyne incognita. . In field inoculation experiments at the end of the first vegetation period, P. vulnus population densities were lower under pistachio than under Prunus or Juglans but there were numerical trends among UCB1 clones. The UCB1 clone with the lowest (UCB1 D71) and the one with numerically the highest (UCB1 D90) population densities of P. vulnus were chosen for microplot experiments in sand and sandy loam soil where they were exposed to different population densities of P. vulnus. . In the fifth year of the field inoculation experiments, P. vulnus population densities surged to levels comparable to those under other nut crops illustrating the principal susceptibility of Pistacia spp. In the sand and sandy loam soil microplots, UCB1 D71 grew more vigorously than UCB1 D90, but both grew less at increasing P. vulnus population densities. In a repeat of this experiment, higher population densities of P. vulnus at planting resulted in thinner plants at the end of the second growing season. A population density of 13.3 to 13.6 P. vulnus 250 cm-3 was determined as the tolerance limit for damage by P. vulnus on UCB1. Low initial population densities of P. vulnus developed slowly on pistachio but medium population densities at planting damaged UCB1 rootstocks when planted to P. vulnus- infested soil. This called for caution when pistachio are planted into soil known to have remedial population densities of P. vulnus from previous crops.