The characteristics of mobile genetic elements in bifidobacteria are not well understood. We characterized an insertion sequence-like element of the IS200/IS605 family found in a size-increased cryptic plasmid in Bifidobacterium longum. During a plasmid profile analysis of B. longum BK strains, we encountered a 6.5-kbp cryptic plasmid pBK283 in B. longum BK28, the size of which has not been identified in bifidobacteria. Nucleotide sequence analysis indicated that an insertion sequence-like element was inserted into the 5.0-kbp pKJ50-like plasmid and resulted in a size increase of pBK283. The element, named ISBlo15, was 1593 bp in length and contained a single ORF encoding a putative transposase, which is similar to the transposase OrfB encoded by IS200/IS605 family elements. Several sequence characteristics, including conserved transposase motifs in OrfB and terminal palindromic sequences that differ from the typical terminal inverted repeats, strongly suggested that ISBlo15 is a member of the IS200/IS605 family. Sequences similar to ISBlo15 were widely distributed among the nine Bifidobacterium species tested, and those of highly homologous sequences were detected only in Bifidobacterium gallicum JCM8224(T). (C) 2010, The Society for Biotechnology, Japan. All rights reserved.