Aeromonas hydrophila AH-1N shows biphasic growth in glucose containing broth at 30°C. The first phase is characterized by strong acidification, while the second phase is accompanied by an increase in medium pH. Disruption of AHL production by knockout of ahyI did not affect the first growth phase, but resulted in further acidification which completely blocked the second growth phase and even caused cell death. The block could be relieved by addition of 5 mM (or less) synthetic N-butanoyl-L-homoserine lactone (C4-HSL), the major AHL produced by A. hydrophila AH-1N. Further analysis revealed that, as shown previously for Serratia plymuthica, butanediol fermentation is regulated by AHL-mediated quorum sensing in A. hydrophila AH-1N. These results indicate that A. hydrophila switches to butanediol fermentation to reduce medium acidification under certain conditions and to allow further growth if nutrient resources are not yet exhausted.