Aims: To calculate the energetic requirements for benzoic acid tolerance in Zygosaccharomyces bailii in chemostat experiments. Methods and Results: A 5.6-l stirred-tank chemostat was used. The yield of ATP (Y-ATP) was calculated under nitrogen atmosphere, assuming equimolar ATP and ethanol production. Under these conditions Y-ATP, equal to 20 g mol(-1) of ATP, was not affected by the acid, whereas the maintenance coefficient (m(ATP)) increased from 1.0 mmol of ATP g(-1) h(-1) in the absence of the acid to 4.8 in the presence of 0.67 mmol l(-1) undissociated benzoic acid. These ATP requirements were similar to those found in Saccharomyces cerevisiae with other weak acids. Conclusions: No significant differences have been found in the energy expended to cope with the acid between sensitive and tolerant species. Therefore, the main difference between tolerant and sensitive species could rely on cellular features that would not need extra energy in terms of ATP. Significance and Impact of the Study: The potential mechanisms involved in the tolerance to weak acids in yeasts have been extensively studied but their actual relevance has not been assessed. Our results suggest that future efforts should concentrate on nonexpending energy features as membrane permeability and metabolic tolerance in the cytoplasm.