The Western flower thrips, Frankliniella occidentalis (Pergande), is a major pest, causing serious damage on eggplant in greenhouses. The present study was conducted to determine the efficacy and optimum release rate of the predators Onus niger Wolff and O. laevigatus Fieber in terms of their performance post-release in Antalya (Turkey) during 2003-2005. The predators were released (1, 2, 4 and 6 adults of each Onus per m(2)) only one time at the beginning of growing season in the first year. However, suppression of the thrips by Orius was not sufficient, particularly at the beginning of spring. Therefore, a second release was performed in the spring period in the second year trials. The findings showed O. laevigatus to be a more efficient predator in controlling F. occidentalis than O. niger in greenhouses. O. laevigatus displayed more rapid colonization than O. niger and efficiently controlled F. occidentalis on eggplant after two releases of 6 adults per m(2) in October and March, whereas O. niger exhibited slower colonization of the plants compared to O. laevigatus, and its predatory effect on the pest thrips took longer.