A derivative of the IS903 transposon (Tn) is described that is capable of creating lacZ gene fusions upon transposition. It should find wide use as a tool for Tn mutagenesis in bacteria since it can be used both to generate mutants and to examine gene expression. The transposase-encoding gene (tnp) is located outside the Tn in the vector, thus Tn insertions into a genome are stably maintained in the absence of its cis-acting transposase (Tnp). The element carries a KmR gene allowing for the direct selection of transposition events in hosts that cannot support pBR322 plasmid replication and facilitating the subcloning of genes into which the Tn has inserted.