The blue-green phenazine, Pyocyanin (PYO), is a well-known virulence factor produced by Pseudomonas aeruginosa, notably during cystic fibrosis lung infections. It is toxic to both eukaryotic and bacterial cells and several mechanisms, including the induction of oxidative stress, have been postulated. However, the mechanism of PYO toxicity under the physiological conditions of oxygen limitation that are encountered by P. aeruginosa and by target organisms in vivo remains unclear. In this study, wild-type and mutant strains of the yeast Saccharomyces cerevisiae were used as an effective eukaryotic model to determine the toxicity of PYO (100-500 μmol/L) under key growth conditions. Under respiro-fermentative conditions (with glucose as substrate), WT strains and certain H2 O2 -hypersensitive strains showed a low-toxic response to PYO. Under respiratory conditions (with glycerol as substrate) all the strains tested were significantly more sensitive to PYO. Four antioxidants were tested but only N-acetylcysteine was capable of partially counteracting PYO toxicity. PYO did not appear to affect short-term respiratory O2 uptake, but it did seem to interfere with cyanide-poisoned mitochondria through a complex III-dependent mechanism. Therefore, a combination of oxidative stress and respiration disturbance could partly explain aerobic PYO toxicity. Surprisingly, the toxic effects of PYO were more significant under anaerobic conditions. More pronounced effects were observed in several strains including a 'petite' strain lacking mitochondrial DNA, strains with increased or decreased levels of ABC transporters, and strains deficient in DNA damage repair. Therefore, even though PYO is toxic for actively respiring cells, O2 may indirectly protect the cells from the higher anaerobic-linked toxicity of PYO. The increased sensitivity to PYO under anaerobic conditions is not unique to S. cerevisiae and was also observed in another yeast, Candida albicans.