Field trials were conducted to determine the attractiveness of aqueous formulations of a protein bait, NuLure, as compared with a standard bait of pelletized torula yeast plus sodium borate for the Mediterranean fruit fly, Ceratitis capitata (Weidemann); the Mexican fruit fly, Anastrepha ludens (Loew); and other Anastrepha spp. The addition of 1-10% borax to 10% NuLure solution increased bait pH, which corresponded directly with increase in number of female C. capitata trapped in field trials. The percentage of female C. capitata trapped with NuLure at a pH of 8.6 was significantly greater than with traps baited with five torula yeast plus borax pellets. NuLure + 1% borax attracted significantly fewer male C. capitata than either torula yeast plus borax pellets or NuLure + 5 or 10% borax. Significantly fewer female and male A. ludens were attracted by the NuLure + 1% borax than most of the other baits tested. Capture of female and male flies of other Anastrepha spp. generally paralleled the preference in attraction of A. ludens to the various protein baits.