The transfer ribonucleic acids (tRNAs) of B. subtilis at different growth phases are examined for changes in the composition and the methylation of minor constituents. The composition of the tRNAs indicates about equal amounts of adenosine and uridine, and of guanosine and cytidine. About 3-4 residues are present as modified bases in the average tRNA molecule. The net composition of tRNAs appears to remain unaltered during different growth phases. In vitro methylation of tRNAs indicates lack of methyl groups in both exponentially growing cells and spores. In vivo methylation studies show tRNA methylation occurs during the stationary phase in the absence of net tRNA synthesis. Thus, both in vitro and in vivo methylation indicate that the tRNAs in exponentially growing cells do not contain their full complement of modified bases. More complete modification is noted in tRNAs from stationary cells or spores. Hence, tRNA mofifications in general are preserved with fidelity even in the dormant spore but the possibility is left open that specific modifications of selected isoacceptors of tRNAs may occur.