Yeast tRNA-Phe has been labelled with 125-I under conditions which conserve the tertiary structure. Significant labelling was only found to occur on specific cytidines in single stranded regions, while other cytidines in single stranded regions and all those in the double stranded region underwent iodination to a very small extent. The pattern obtained from iodine labelling satisfies the conformation of a model recently proposed for this tRNA.