Seven transfer ribonucleic acid (tRNA) methylase mutants were isolated from Escherichia coli K-12 by examining the ability of RNA prepared from clones of unselected mutagenized cells to accept methyl groups from S-adenosylmethionine catalyzed by crude enzymes from wild-type cells. Five of the mutants had an altered uracil-tRNA methylase; consequently their tRNA's lacked ribothymidine. One mutant had tRNA deficient in 7-methylguanosine, and one mutant contained tRNA lacking 2-thio-5-methylaminomethyluridine. The genetic loci of the three tRNA methylase mutants were distributed over the E. coli genome. The mutant strain deficient in 7-methylguanosine biosynthesis showed a reduced efficiency in the suppression of amber mutations carried by T4 or lambda phages.