Background: An excess of caffeine is cytotoxic to all eukaryotic cell types. We aim to study how cells become tolerant to a/ntoxic dose of this drug, and the relationship between caffeine and oxidative stress pathways./nMethodology/Principal Findings: We searched for Schizosaccharomyces pombe mutants with inhibited growth on caffeinecontaining/nplates. We screened a collection of 2,700 haploid mutant cells, of which 98 were sensitive to caffeine. The genes mutated in these sensitive clones were involved in a number of cellular roles including the H2O2-induced Pap1 and Sty1 stress pathways, the integrity and calcineurin pathways, cell morphology and chromatin remodeling. We have investigated the role of the oxidative stress pathways in sensing and promoting survival to caffeine. The Pap1 and the Sty1 pathways are both required for normal tolerance to caffeine, but only the Sty1 pathway is activated by the drug. Cells lacking Pap1 are/nsensitive to caffeine due to the decreased expression of the efflux pump Hba2. Indeed, ?hba2 cells are sensitive to caffeine, and constitutive activation of the Pap1 pathway enhances resistance to caffeine in an Hba2-dependent manner. Conclusions/Significance: With our caffeine-sensitive, genome-wide screen of an S. pombe deletion collection, we have/ndemonstrated the importance of some oxidative stress pathway components on wild-type tolerance to the drug.