Current ammonia-based lures vary considerably in their ability to attract Anastrepha fruit flies in the field, and this variability does not appear to be related solely to ammonia release rate. Electroantennography (EAG) and flight tunnel bioassays were used to examine the effect of age on fly response to ammonia and carbon dioxide, two volatiles released from commercial ammonium bicarbonate lures. EAG measurements from female Caribbean fruit flies, A. suspensa (Loew), showed that ammonia generated a larger amplitude EAG response in sexually immature flies than in mature flies. Conversely, carbon dioxide elicited stronger EAG responses in mature females. In flight tunnel bioassays, both age groups responded positively to ammonia in doses ranging from 0.06-3.84 mg/h, but captures of immature flies declined with increasing ammonia concentration, suggesting repellency at high doses for the immature group. Carbon dioxide, ranging from 5-120 μg/h, did not capture any flies when presented alone in the flight tunnel. However, carbon dioxide in combination with ammonia was more attractive than ammonia alone, but only for sexually mature flies. These age-related differences in response to ammonia and carbon dioxide may account for some of the variability observed in field tests with ammonium bicarbonate lures.