Host—selection behavior was studied in the mycophagous species, Drosophila falleni, D. putrida, and D. testacea. By marking flies with fluorescent dusts and recapturing them, it was determined that individuals of each of the 3 Drosophila species are attracted to >1 species of mushroom. There appeared to be little variation among individuals in their preferences for the different species of mushrooms. The polyphagy of individuals is probably selected favored by the unpredictability of fructification of individual mushroom species.