Nine major transfer RNA (tRNA) gene clusters were analysed in variousVibrio cholerae strains. Of these, only the tRNA operon I was found to differ significantly inV. cholerae classical (sixth pandemic) and El Tor (seventh pandemic) strains. Amongst the sixteen tRNA genes contained in this operon, genes for tRNA Gln3 (CAA) and tRNA Leu6 (CUA) were absent in classical strains as compared to El Tor strains. The observation strongly supported the view that the above two pandemic strains constitute two different clones.