We have used tRNAs protected against periodate oxidation by charging with specific amino acids to show that Escherichia coli valyl-tRNA synthetase ( l-valine : tRNA ligase (AMP), EC 6.1.1.9) can charge several Neurospora crassa tRNAs, including tRNA Phe. Maximum heterologous charging required the presence of dimethylsulfoxide and large concentrations of synthetase. In a mixed N. crassa tRNA preparation, E. coli valyl-tRNA synthetase charged primarily tRNAs other than tRNA Phe; but in a preparation of tRNA Phe further purified about 11-fold on benzoylated DEAE-cellulose, tRNA Phe was the predominant tRNA charged. We conclude if a number of tRNAs can be charged in a heterologous system such as this, then such reactions provide a limited insight into the mechanism of specific recognition of tRNAs by aminoacyl-tRNA synthetases.