Nuclei and enucleated Xenopus laevis oocytes were used as a source of exonucleases that hydrolyze the 3'terminus sequence of tRNA. The majority of the total activity was found in the cytoplasm. Several forms of enzyme activity are obtained upon fractionation in S-200 column. Both nuclear and cytoplasmic activities display a preferential cleavage of the terminal AMP residue. Similar results are obtained in vivo by microinjection of tRNA labelled in the CCA sequence. Aminoacylation of tRNA, on the other hand, protects the terminal nucleotide against exonuclease activity.