Résumé : We have determined the nucleotide sequence of the major species of E.colitRNA Ser and of a minor species having the same GGA anticodon. These two tRNAs should recognize the UCC and UCU codons, the most widely used codons for serine in the highly expressed genes of E.coli. The two sequences differ in only one position of the D-loop. Neither tRNA has a modified adenosine in the position 3′-adjacent to the anticodon. This can be rationalized on the basis of a structural constraint in the anticodon stem and may be related to optimization of the codon-anticodon interaction. Conparison of all E.coli serine tRNAs (and that encoded by bacteriophage T 4) reveals characteristic (possibly functional) features. Evolutionary analysis suggests an eubacterial origin of the T 4tRNA Ser gene and the existences of a recent common ancestor for thetRNAGUCSerandtRNAGUCSergenes. © 1985 IRL Press Limited.