Two single-base mutations in 16S rRNA conferring high-level resistance to spectinomycin were isolated on a plasmid-borne copy of the rrnD operon from Salmonella enterica serovar Typhimurium. Neither of the mutations (C 1066U and C 1192U) had appreciable effects on cell growth, but each had differential effects on resistance to spectinomycin and fusidic acid. Both mutations also conferred resistance to spectinomycin in Escherichia coli strains containing deletions of all seven chromosomal rrn operons and expressing plasmid-encoded Salmonella rRNA exclusively. In contrast, when expressed in E. coli strains containing intact chromosomal rrn operons, the strains were sensitive to spectinomycin. However, chromosomal mutations arose that allowed expression of the rRNA-dependent spectinomycin resistance phenotype. It is proposed that in heterogeneous rRNA populations, the native E. coli rRNA out-competes the heterologous Salmonella rRNA for binding to ribosomal proteins, translation factors, or ribosome assembly, thus limiting entry of the antibiotic-resistant 30S subunits into the functioning ribosome pool.