The spatial-temporal population dynamics of an imported parasitoid of stemborers, Cotesia flavipes Cameron, were analyzed. A two-step spatial interpolation method, spatial splines with Kriging of the residuals, was developed for interpolating the distribution of C. flavipes in Kenya. The results indicate that C. flavipes has become established throughout the whole southern part of Kenya and northern part of Tanzania, and that the population density of C. flavipes in maize fields is still increasing. The maximum C. flavipes density was about one parasitized borer per plant in the first growing season of 1999. The mean C. flavipes densities varied from region to region in 1999, with the highest density (five parasitized borers per 20 plants) in the southeast and the lowest parasitism in central Kenya (one parasitized borer per 20 plants). The percentage of sites occupied by C. flavipes was 90% in the southeastern area during the first growing season of 1999. In coastal Kenya, C. flavipes occupied >70% of the sampling sites, compared with ≈50% in both central and western Kenya. Over the whole sampling area, C. flavipes increased its distribution from 60% of sites occupied in 1994–80% in 1999. Temporally, C. flavipes population density remained low, with an average of less than one parasitized borer per 20 plants, until the second growing season of 1997–1998, when there was a sharp increase. The suppression of stemborer populations by C. flavipes appeared in southeastern and coastal Kenya from 1998. Analyses indicated that C. flavipes was a minor parasitoid before 1995, but had become the predominant parasitoid after 1998. The results of the spatial interpolation showed that the C. flavipes population did not spread much from the release sites before the first growing season of 1996, but expanded tremendously after the first growing season of 1997. The spatial interpolation model was validated with field data from 1999. The model predicted the C. flavipes density well at the zonal level but underestimated C. flavipes density country-wide. The possible displacement of an ecologically similar native congener, C. sesamiae, is discussed.