Although protein synthesis usually terminates when a stop codon is reached along the messenger RNA sequence, there are examples, mainly in viruses, of the stop codon being suppressed by a tRNA species. A strong candidate for this phenomenon occurs in tobacco mosaic virus (TMV) in the form of two proteins (110K and 160K, of molecular weights 110,000 and 160,000, respectively)1, sharing an N-terminus sequence, which are translated in vitro from a purified species of viral RNA. We have investigated the identity of the tRNA responsible for production of the 160K protein and show here that it is one of the tyrosine tRNAs. Another tyrosine tRNA, in which the first base of the anticodon is highly modified, does not act as a suppressor, indicating the possible regulatory function of such modifications.