Adult alfalfa weevils, Hypera postica (Gyllenhal), showed a strong olfactory attractancy to fresh alfalfa plant material in an olfactometer, but dried material did not attract them. Responses were strongest when materials were positioned above the insects. Age of the adults and pretest holding period in the refrigerator appeared not to affect the olfactory response; a 2-day pretest starving period elicited a higher response than did any other starving period tested. Fresh alfalfa plant material was extracted with steam distillation, ether extraction, and fractional distillation, and was watered down to dilutions of 0.1%, 0.05%, 0.025%, and 0.0125% by volume. Of these dilutions, that at 0.025% elicited a miximum response, but this response declined markedly when the insects were offered water before they were exposed to the extract. Addition of a 2% sugar solution to the 0.025% extract dilution elicited a higher overall response from the weevils than did any other test substance, including even alfalfa. Weevils with both antennae removed did not respond predictably when exposed to alfalfa vapors in the olfactometer. This phenomenon was thought to result from loss of both geotactic and chemotactic centers of the antennae.