Yellow-emitting Vibrio fischeri Y1 modulates its bioluminescence (BL) depending on the dissolved O₂ concentration. On supplying O₂ to the cells under anaerobiosis, the cells begin to emit striking yellow BL peaking around 535 nm. The enhanced yellow emission reverts reversibly to the original level after O₂ is consumed. Moreover, the reversible rise and fall of the yellow emission occurs repeatedly in accord with the repeating cycles of aeration on and off. This indicates that an increase in the cellular amount of yellow fluorescent protein (YFP) is not an immediate cause of the yellow emission enhancement. One suggested explanation is that the activity of YFP originating from its highly fluorescent property is altered by redox interaction with the respiratory components, including the soluble cytochrome c. Under the O₂-limited conditions, the cellular YFP molecules, in part, seem to lose the fluorescent property possibly because of being reduced via redox interaction with some respiratory components in reduced form. On stimulating aerobic respiration with O₂ supply, the reduced YFP seems to retrieve its fluorescent property via oxidation possibly with both O₂, diffused across the cell membrane, and ferricytochrome c, generated during the respiratory turnover. The suggested redox interactions seem primarily to cause the reversible BL modulation.