Host-finding behavior by the onion fly,Hylemya antiqua(Meigen), was observed in the field in response to sliced onions (the host plant), barnyard grass (Echinochloasp.), and a control treatment. More flies landed within 0.5 m of the onion bait than within 0.5 m of any of the other treatments. Flies which landed downwind of the onion bait flew directly or in a series of short flights toward the volatile source. These behaviors were not observed in the vicinity of the other treatments. Traps designed to assess flight direction with respect to the wind substantiated the observations of female and male upwind flight in response to distant host-plant odors. These results show that onion flies locate host plants as a result of positive taxes, of which anemotaxis plays a significant role.