Transketolase mutants have been selected after ethyl methane sulfonate mutagenesis of Escherichia coli. These strains are unable to grow on any pentose and, in addition, require a supplement of aromatic amino acids or shikimic acid for normal growth on any other carbon source. Revertants are normal in both respects and also contain transketolase. Transketolase mutants do not require exogenous pentose for growth. Preliminary genetic mapping of the locus is presented.