Abs:ract : The seasonal sequences of species of Culicidae, Simuliidae, Tabanidae and several other bloodsucking Diptera as determined by CO&rap c01kcti0n.s are reported, as are the dates of first flowering of a number of plant species. Insect species that appeared th- p earliest as bloodseeking females were the mosquitoes, Cuiiseta inoranta and Ano@& punctipennis, and the black flies, Cnephia tamiatifrons and Simulium johannseni, each of which was active before the first flowering of such earlyflowering plants as Juneberry, wild plum and pin cherry. Considered as groups, mosquitoes of genera that mostly hibernate as adults (Culiseta, Ano@&s, C&x) were first to appear zs bloodseeking females followed closely by the Simuliidae, then the eariier aedine mosquitoes, and, finally, the Tabanidae and Ceratopogonidae. Among the Tabanidae, species of Chqsops and f&/bon&a appeared much earlier than species of Tabanxc. In 1965 a trapping program for bloodsucking Diptera was started as part of a study of the host relationships of bloodsucking arthropods, their roles in the ecology of arboviruses in Wisconsin, and as pests in recreational and agricultural lands. In this program we had,the following objectives: 1. To determine the seasonal sequence of bloodseeking adults of the families Culicidae, Ceratopogonidae, Simuliidae, and Tabanidae. Such information, in conjunction with data on host-preferences, would be useful both in determining with which arthropod species a vertebrate might have had contact before acquiring an arbovirus infection, and in revealing the sequence of problems posed by the various species as biting pests in recreational areas. 2. To determine how closely the seasonal appearances of the adults parallel those of flowering and other phenological botanical events. Many variables are involved. But if a predicatable parallel exists, easily observed plant events might serve as useful indicators of the progress of the season, not only for entomologists, but also for nonentomologists such as park and forest administrators who must deal with problems posed by biting insects. 3. To establish a continuing surveillance of arboviruses in arthropods-this surveillance to