To study the influence of individual attractants and coattractants, their concentrations,pH, and age-efficacy relationship, various attractant mixtures were prepared and bioassayed against field populations of the eye gnat,Hippelates collusor(Townsend). An aqueous mixture of trimethylamine, ammonia, linoleic acid, indole, and acetic acid showed the greatest attractancy atpH 6.0. All these compounds were essential in inducing a high level of response in these insects. A simultaneous increase in the concentrations of trimethylamine, ammonia, indole, and acetic acid did not increase the attractancy. On the contrary, the attractancy significantly declined at very high concentrations. Aging of attractant mixtures at various strengths did not affect activity at 26–28°C for 72 h, but the attractancy of the mixtures decreased somewhat at 32.2°C after 24–120 h. Studies on the effects of the concentration of each attractant and coattractant on the activity showed that optimum concentrations of trimethylamine, ammonia, linoleic acid, and indole were 0.1–5.0, 0.5–5.0, 0.01–0.05, and 0.001–0.01%, respectively.n-Butyric acid at 1% concentration also enhanced the attractancy. This information is basic to the development of potent synthetic attractant formulations for the control ofHippelateseye gnats.