The kanamycin resistance transposon Tn2680, which originates from the R plasmid Rts1, is homologous to Tn6 and carries two directly repeated copies of IS26, one at each end. The kanamycin resistance gene codes for type I aminoglycoside-3'-phosphotransferase. Tn2680 also contains, in the middle of the transposon, an additional IS element homologous to IS903. This element, designated IS903.B, is flanked by a 9-base-pair direct target duplication. A novel kanamycin resistance transposon. Tn2681, can be generated from Tn2680 by IS903.B-mediated cointegration and subsequent reciprocal recombination between the directly repeated IS26 sequences. Tn2681 carries a single IS26 element in the middle of the transposon and is flanked by two directly repeated copies of IS903.B. Possible evolutionary relationships between Tn2680 and other kanamycin resistance transposons such as Tn903 and Tn2350 are discussed, based on the gene organization and DNA sequences.