We found that Y base from tRNAPhe showed a positive CD peak at 327 nm, and that its intensity varied in accordance with the concentration of Mg2+ or spermidine. Similar changes were observed in the fluorescence spectra and it was deduced that such changes are due to a conformational change probably in the region of the anti-codon loop. CD and fluorescence spectra of phenylalanyl-tRNAPhe were also measured and the conformational change of tRNA accompanying amino-acylation was discussed. The apparent binding constant of phenylalanyl-tRNAPhe with poly U was estimated to be 4.4×lO3M−1 by the fluorescence technique.