Few methods have been described for marking tabanids. Bennett and Smith (1968) marked them with32p in water which the flies imbibed. Thornhill et al. (1971), and Thornhill and Hays (1972) marked flies with model airplane dope. Face flies,Musca autumnalisDe Geer, were marked successfully with fluorescent powder for field studies (Turner and Gerhardt 1965). In studying tabanid flight in Louisiana we wanted to mark large numbers of flies with a minimum of interference. We allowed the flies to mark themselves with Day-Glo fluorescent pigments3as they passed through a selfmarking device mounted on a canopy trap (Catts 1970), as modified by Adkins et al. (1972).