The mechanism of codon recognition by tRNA is investigated in the system tRNAPhe + UUC by temperature-jump measurements using the Wye base fluorescence as a label. In 0.4 M Na+ and 5 mM Mg2+ a two-step reaction is observed and described quantitatively; UUC is shown to bind preferentially to one of two conformations on the anticodon loop. In 0.1 m Na+ and 10 mM Mg2+ an additional relaxation effect is observed, which indicates a codon-induced conformation change leading to an association of tRNA molecules. The codon-induced tRNA association is demonstrated independently by equilibrium sedimentation. The present results suggest a more active role of tRNA during translation than anticipated.