Ethanol-hypersensitive strains (ets mutants), unable to grow on media containing 6% ethanol, were isolated from a sample of mutagenized Schizosaccharomyces pombe wild-type cells. Genetic analysis of these ets strains demonstrated that the ets phenotype is associated with mutations in a large set of genes, including cell division cycle (cdc) genes, largely non-overlapping with the set represented by the temperature conditional method; accordingly, we isolated some ets non-ts cdc- mutants, which may identify novel essential genes required for regulation of the S. pombe cell cycle. Conversely, seven well characterized ts cdc- mutants were tested for their ethanol sensitivity; among them, cdc1-7 and cdc13-117 exhibited a tight ets phenotype. Ethanol sensitivity was also tested in strains bearing different alleles of the cdc2 gene, and we found that some of them were ets, but others were non-ets; thus, ethanol hypersensitivity is an allele-specific phenotype. Based on the single base changes found in each particular allele of the cdc2 gene, it is shown that a single amino acid substitution in the p34cdc2 gene product can produce this ets phenotype, and that ethanol hypersensitivity is probably due to the influence of this alcohol on the secondary and/or tertiary structure of the target protein. Ethanol-dependent (etd) mutants were also identified as mutants that can only be propagated on ethanol-containing media. This novel type of conditional phenotype also covers many unrelated genes. One of these etd mutants, etd1-1, was further characterized because of the lethal cdc- phenotype of the mutant cells under restrictive conditions (absence of ethanol). The isolation of extragenic suppressors of etd1-1, and the complementation cloning of a DNA fragment encompassing the etd1+ wild-type gene (or an extragenic multicopy suppressor) demonstrate that current genetic techniques may be applied to mutants isolated by using ethanol as a selective agent.