We tabulated plant species that served as floral hosts of adult Syrphidae and Tachinidae (Diptera) as reported by C. Robertson in his 33-yr data set of flower-visiting insects of central Illinois. Of the 186 fly species he recorded, most were visiting species of the Asteraceae and Apiaceae. The most preferred plant species were the asteraceous Aster pilosus Willdenow and Heracleum maximum Bartram and the umbellifer Pastinaca sativa L. The most frequently recorded fly species were three syrphids: the aphidophagous Toxomerus marginatus (Say) and Sphaerophoria contiqua Macquart and the detritivorous Syritta pipiens (L.). Most fly species evidently visited only a few plant species. These findings may have important implications for research in pollination ecology and insect behavior and for use of fly species as agents of conservation biological control.