The present work suggests that a significant proportion of spontaneous mutations in Escherichia coli are the result of translational errors. This idea is supported by the following observations: (i) Streptomycin can induce the formation of auxotrophic mutants in streptomycin-sensitive cells, but not in rpsL mutants resistant to streptomycin, and (ii) strains having hyperaccurate ribosomes (rpsL999 and rpsL1204 strains) show reduced mutation rates. The implications of these results are discussed with respect to the dogma of randomness of spontaneous mutations and the directed mutation hypothesis.