A BSTRACT Laboratory bioassays enabled us to determine the mortality of Argentine ant ( Linepithema humile [Mayr]) workers, and red imported fire ant ( Solenopsis invicta Buren) workers exposed to sodium hydrogen carbonate (NaHCO 3 , sodium bicarbonate). The median lethal concentration (LC 50 ) of NaHCO 3 for Argentine ants was 5.64 mg per cm 2 after 5 d exposure and 3.96 mg per cm 2 after 6 d. Cumulative mortality for Argentine ants exposed to 28 mg NaHCO 3 per cm 2 was 89.5% on day 6. Workers

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... of both species were exposed to concentrations of 9.92, 17.70, or 152.00 mg NaHCO 3 per cm 2 in separate tests. Mortality of Argentine ants was significantly higher than that of fire ants following exposure to 9.92 mg NaHCO 3 per cm 2 , while mortality for the two species did not differ following exposure to the two higher concentrations. Mortality of both species treated with the highest concentration exceeded 99% at 6 d. In tests with equivalent amounts of sodium in NaHCO 3 and NaCl treatments, mortality for fire ants exposed to NaHCO 3 was about 46% after 6 d. Mortality for fire ants exposed to NaCl was about 15% and was similar to that for untreated ants. Argentine ants were provided sugar water baits containing a range of NaHCO 3 concentrations. Argentine ant mortality after 6 d exposure to 5% NaHCO 3 -sugar water treatment was about 50%. Mortality was not higher for workers exposed to higher concentrations of NaHCO 3 in sugar water baits. Enzymatic dysfunction caused by unfavorable increases in internal pH is the most likely explanation for worker mortality following exposure to NaHCO 3 .