||Based on the high acid tolerance and the simple nutritional requirements of Saccharomyces cerevisiae, engineered strains of this yeast are considered biocatalysts for industrial production of high-purity undissociated lactic acid. However, high concentrations of lactic acid are toxic to S. cerevisiae, thus limiting its growth and product formation. Physiological and transcriptional responses to high concentrations of lactic acid were studied in anaerobic, glucose-limited chemostat cultures grown at different pH values and lactic acid concentrations, resulting in a 50% decrease in the biomass yield. At pH 5, the yield decrease was caused mostly by osmotically induced glycerol production and not by the classic weak-acid action, as was observed at pH 3. Cultures grown at pH 5 with 900 mM lactic acid revealed an upregulation of many genes involved in iron homeostasis, indicating that iron chelation occurred at high concentrations of dissociated lactic acid. Chemostat cultivation at pH 3 with 500 mM lactate, resulting in lower anion concentrations, showed an alleviation of this iron homeostasis response. Six of the 10 known targets of the transcriptional regulator Haa1p were strongly upregulated in lactate-challenged cultures at pH 3 but showed only moderate induction by high lactate concentrations at pH 5. Moreover, the haa1Delta mutant exhibited a growth defect at high lactic acid concentrations at pH 3. These results indicate that iron homeostasis plays a major role in the response of S. cerevisiae to high lactate concentrations, whereas the Haa1p regulon is involved primarily in the response to high concentrations of undissociated lactic acid.