Summary Of 48 temperature-sensitive mutants of Saccharomyces cerevisiae examined, five belonging to the same complementation group were found to undergo extensive loss of viability at the restrictive temperature. These mutants were protected from the lethal effects of exposure to a non-permissive temperature by starving for an auxotrophic requirement. By analogy with the method described by Littlewood  for selecting antibiotic-sensitive mutants, these temperature-sensitive mutants were found suitable in enriching for specific auxotrophs. Optimal conditions have been determined for selecting specific auxotrophs after mutagenesis by N-methyl- N′-nitro- N-nitrosoguanidine. These enable 20-fold enrichment and at least in the case of mutation to adenine dependence the method does not appear to favour mutations at any particular locus.