The nucleotide sequence of repeated sequence I, which appears in five regions on nylon oligomer-degrading plasmid pOAD2, harbored in Flavobacterium sp. strain K172, was determined. The five regions of repeated sequence I had 880 bp of identical sequence, and the sequence was identical to that of IS6100, an insertion sequence classified in the IS6 family, initially found in Mycobacterium fortuitum. Sequences homologous to that of IS6100 were found for another nylon oligomer-degrading plasmid, pNAD2, harbored in Pseudomonas sp. strain NK87, by Southern hybridization experiments.