A streptomycin-resistant mutantant of Bacillus subtilis that is also asporogenous, was isolated and partially characterized. This strain, SRB15, sporulated at a frequency of about 1% compared to the wild type frequency of greater than 70%. The two phenotypes were inseparable by transformation, suggesting that this strain carries a single mutation that causes it to be both streptomycin-resistant and spore-minus. The mutation cotransduces with cysA, the closest auxotrophic marker to the "ribosomal region" of the B. subtilis chromosome, with a frequency of 68%. SRB15 showed no cross resistence to other antiobiotics tested, including the aminoglycosides kanamycin, neomycin and spectinomycin. Ribosomes obtained from the mutant were at least 200-fold more resistant in vitro to streptomycin than were wild type ribosomes in the translation of phage SPO1 RNA. The kinetics of in vitro translation of this natural message were indistinguishable for mutant and wild type ribosomes. The level of misreading, as measured by poly(U)-directed isoleucine incorporation, by mutant ribosomes was less than that by wild type ribosomes.