1. Amino acid activating enzymes (aminoacyl tRNA ligases) specific for valine and leucine tRNA have been partially purified from four widely differing phyla, namely yeast, E. coli, starfish and toadfish. 2. The rates of aminoacylation of the four tRNA's have been determined using both the homologous enzyme and the three heterologous enzymes. In most cases there was appreciable cross-reaction, the E. coli enzyme and tRNA being in general least disposed to cross-react with the others. 3. In some cases the heterologous pair react more rapidly than the homologous. In one case this has been established as being a consequence of a much higher Vmax that overcomes a poorer enzyme tRNA association. 4. Both homologously and heterologously esterified tRNA's appear to be homogeneous as measured by the kinetics of hydrolysis. 5. We interpret these observations as indicating that all tRNA's specific for a particular amino acid have at least one common polynucleotide sequence, probably in an area of the chain which is not base-paired. The marked differences in the rates of esterification and in the rates of hydrolysis of the aminoacyl tRNA's reflect other structural variations in tRNA's that do not include the enzyme recognition site. In particular E. coli enzymes are least able to adapt themselves to heterologous tRNA's.