Weak luminescence was detected from oxygenated liquid cultures of the yeast Candida utilis during two stages of its growth cycle. The first period of emission occurred during the exponential phase of growth and comprised an ultraviolet band (270-390 nm; ca 19 photons s-1 cm-2 of culture surface) and a visible band (450-620 nm; ca 68 photons s-1 cm-2). The second period of emission occurred late in the stationary phase of growth and was comprised almost entirely of a visible region band (450-620 nm; 6.8 x 10(2) photons s-1 cm-2). No luminescence was observed when the yeast was grown anaerobically. These observations are compared with those previously obtained for two other yeasts, Saccharomyces cerevisiae and Schizosaccharomyces pombe. The ratios of the intensities of the blue/red emissions in the stationary phase luminescences correlated with the ratio of the saturated/unsaturated lipid content for the three yeasts. This result provided further support for the claim that the stationary phase luminescence arises from the reactions associated with lipid peroxidation. A number of previously suggested sources of the exponential phase luminescence are discussed and rejected. Oxidative side reactions accompanying protein synthesis remain a possible source of that emission.