flies (Diptera: Bombyliidae) are ectoparasitoids of larval stages of insects, often digger bees and wasps. We studied the behavior of 4 species of the genus Bombylius at a nest aggregation of their host bee, Lasioglossum malachurum Kirby, and at an adjacent feeding site. Although eggs were frequently thrown on vegetation patches, the number of eggs oviposited and the time spent in hovering flight were higher at host nest entrances. Bombylius females fed essentially on 3 (2 Caryophyllaceae and 1 Asteraceae) of 9 blooming plant species found in the area. Oviposition and feeding activities had different daily distributions. In general agreement with optimal foraging theories, Bombylius females exhibited the strongest interest in the predicted target, i.e., the host nest, and fed essentially on a few but highly exploited plants in the close vicinity of the host nesting site. http://zoolstud.sinica.edu.tw/Journals/48.2/141.pdf B ee flies (Diptera: Bombyliidae) have a virtually cosmopolitan distribution and are commonly found in warm arid to semi-arid habitats (Hull 1973, Evenhuis and Greathead 1999), where they can form a conspicuous part of the flower-visiting insect fauna (Toft 1983). Adults of larger species are powerful and agile fliers, rivaling hoverflies (Syrphidae) in their ability to hover and move in all directions while in flight. With many species possessing colorful patterns of stripes and spots on the wings and bodies, bee flies are often some of the most striking in appearance of all the Diptera. Larvae of all reared species of Bombyliidae are parasitoids (most often ectoparasitoids) or predators of other insects, primarily the immature stages of the large endopterygote orders of Coleoptera, Hymenoptera, Lepidoptera, Orthoptera, and Diptera. Despite their widespread and common