Sequences of two putative tRNA genes, for serine and leucine, from the archaebacterium Sulfolobus solfataricus contain intervening sequences in the anticodon region. Furthermore, the genes lack encoded CCA 3' termini and are flanked by A + T-rich DNA segments. The introns can both form the same secondary structure, which is a double-helical extension of the anticodon stalk. The resulting structure contains two symmetrically placed 3-base bulge loops, in which are located cleavage sites for the introns. In the one case tested, the gene occurs as a single copy in the genome.