The 3'-CCA end of tRNA Phe from Escherichia coli and Thermus thermophilus was changed to AAA, CCC, UUU and UUA by the stepwise degradation procedure of the 3'-CCA end of tRNA Ph° followed by the ligation with oligoribotrinucleotides. Substrate activity of tRNA~h~, and tRNA~h~ in tRNA aminoacylation was shown, tRNA,~h~, is a bad substrate for E. coli and Th. thermophilus phenylalanyl-tRNA synthetases.Phe tRNAutm has no detectable activity in tRNA aminoacylation. Therefore the nature of the Y-end of tRNA Phe plays an important role in tRNA binding and its substrate efficiency. Nevertheless the CCA sequence at the 3'-end of tRNA Phe does not seem to be an absolute requirement for tRNA aminoacylation.