The pathway of biosynthesis of tyrosine in Escherichia coli was investigated with labeled glucose. C-6 of glucose was incorporated into the ring of tyrosine only in C-2,6 and to the extent of 1.1 atoms. This result is identical with that observed earlier in the ring of shikimic acid, a precursor of tyrosine and several other aromatic metabolites. Similar correspondence between the rings of tyrosine and shikimic acid had already been observed with [1-14C]glucose in yeast. Confirmation is therefore provided for the earlier conclusion, based on the enzymic reactions involved in the formation and function of prephenic acid, that in the conversion of shikimate to tyrosine the ring remains intact, C-1 to C-6 of shikimate corresponding to C-1 to C-6, respectively, of tyrosine. In view of the known origin of the rest of the ring of shikimic acid, these results are in conflict with the observation of other workers that in Aerobacter aerogenes [3,4-14C]glucose gives rise to extensive labeling in three successive atoms, either C-2,3,4 or C-4,5,6, of tyrosine. Certain limitations of the degradation of labeled tyrosine are discussed, and a modification of the procedure is introduced.