An Escherichia coli mutant, ts9, previously reported by Flaks et al. (Cold Spring Harbor Symp. Quant. Biol. 31, 623-631, 1966) to have an electrophoretically altered ribosomal protein, has been further characterized and the altered component has been identified as L7/L12. Although mutant ts9 is temperature sensitive for growth (rts-), the rts and L7/L12 mutations are genetically separable and are both located between argH and rif. The L7/L12 mutation (rpyL) maps very close to relC, mutants of which have a defect in the 50S ribosomal subunit. The gene order is argH-rts-(rpyL,relC)-rif. Protein synthesis directed by bacteriophage lambdacI857S7drifd18 in ultraviolet-irradiated cells indicates that L7/L12, As well as L1, L10, L11, and possibly L8 or L9 are coded by the phage DNA. Our results indicate that rpyL is the structural gene for L7/L12 and that this region of the E. coli chromosome contains a cluster of structural genes for ribosomal proteins.