Abstract: We describe the isolation and preliminary characterization of a set of pleiotropic mutations resistant to the minimum inhibitory concentration of cycloheximide and screened for ts (temperature-sensitive) growth. These mutations fall into 22 complementation groups of cycloheximide resistant ts lethal mutations (crl). None of the crl mutations appears to be allelic with previously isolated mutations. Fifteen of the CRL loci have been mapped. At the nonpermissive temperature (37 degrees), these mutants arrest late in the cell cycle after several cell divisions. Half of these mutants are also unable to grow at very low temperatures (5 degrees). Although mutants from all of the 22 complementation groups exhibit similar temperature-sensitive phenotypes, an extragenic suppressor of the ts lethality of crl3 does not relieve the ts lethality of most other crl mutants. A second suppressor mutation allows crl10, crl12, and crl14 to grow at 37 degrees but does not suppress the ts lethality of the remaining crl mutants. We also describe two new methods for the enrichment of auxotrophic mutations from a wild-type yeast strain.