A nearest-neighbor discriminant analysis based on the frequencies of occurrence of six random amplified polymorphic DNA markers was used to identify individuals from an insecticide-resistant laboratory biotype (SEL) of the walnut aphid parasiteTrioxys pallidusafter release into three California walnut orchards. Releases of SEL individuals were made in a Hanford, CA, orchard in 1988 and in a Gridley, CA, orchard in 1989. Nearest-neighbor analysis of postrelease collections from both the Hanford and Gridley orchards showed a continuing influence of the SEL biotype up to 3 yr after the release. SEL individuals also were released in a Hed Bluff, CA, walnut orchard in 1991. At this orchard, SEL males were recovered from the release area throughout the season of release, but a June 1992 sample indicated that no SEL individuals remained in the orchard. Comparison of observed data at Hanford and Gridley to simulated hybrid data provided evidence that there was interbreeding between the SEL and the native field biotypes. Bioassays indicated that the level of azinphosmethyl resistance in these two orchards was intermediate between the level in the orchards before release and the level in the pure SEL biotype. Therefore, the release of pesticide-resistantT. pallidusin two California walnut orchards significantly increased the level of resistance, but did not completely displace the native populations, despite multiple treatments of postrelease populations with azinphosmethyl.