Abstract: The seasonal fluctuation and distribution of Frankliniella occidentalis (Pergande) and Thrips tabaci Lindeman populations in various developmental stages (larvae I, II and adults) was studied in two crops: cucumber (Cucumis sativus L., F1 Kamaron) and tomato (Lycopersicon esculentum M., F1 Arletta) during April-August 2000. Samples of leaves and flowers were collected from the upper and lower half part of the plants. The population peak of F. occidentalis and T. tabaci in cucumber and tomato crops was observed between 20/5/2000 and 30/5/2000. The distribution data for both Thrips species showed that they prefer cucumber to tomato. Both Thrips species prefer leaves than flowers and the upper half than the lower half part of the plants. Additional laboratory research was conducted to determine the predatory effect of Coccinella septempunctata on both Thrips species. According to number of thrips found on cucumber and tomato leaves, the predatory effect of the ladybird was also determined indicating that, when pest density is high, then predation may be ineffective.