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Three strains of Steinernema feltiae Filipjev (All, Mexican, and Breton strains) and one of Heterorhabditis heliothidis (Khan, Brooks, and Hirschmann) were evaluated for their potential to control Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), larvae and pupae in the soil. In laboratory studies, H. heliothidis and S. feltiae (Mexican strain) produced the highest mortality (6 days posttreatment) of CPB when applied to the surface of a soil column containing mature CPB larvae 5 cm below. Mortality ranged from 80 to 90% at rates of 79-158 nematodes/cm2. Similar results were seen in a field microplot study with all four nematodes; S. feltiae (Mexican strain) and H. heliothidis were most effective. Adult CPB emergence was reduced 86.5-100% after application of 31-93 H. heliothidis/cm2 and 88.4-100% with 93-155 S. feltiae (Mexican strain)/cm2. The All strain of S. feltiae was moderately effective (ca. 80% reduction at 93-155 nematodes/cm2), while the Breton strain was ineffective (< 40% reduction at 155 nematodes/cm2). In small plots of potatoes enclosed in field cages, application of H. heliothidis and S. feltiae (Mexican strain) at rates of 93-155 nematodes/cm2 before larval CPB burial in the soil resulted in 66-77% reduction in adult CPB emergence. Soil applications of these nematodes show potential for biological control of CPB.