Small bacteriophage Mu transposable elements containing the lac operon structural genes were constructed to facilitate the isolation and use of Mu insertions and lac gene fusions. These mini-Mu elements have selectable genes for either ampicillin or kanamycin resistance and can be used to form both transcriptional and translational lac gene fusions. Some of the mini-Mu-lac elements constructed are deleted for the Mu A and B transposition genes and form stable insertions that cannot undergo transposition unless complemented for these functions. A procedure was developed for selecting mini-Mu insertions specifically into plasmids, including commonly used high-copy-number cloning vectors such as pBR322. Mu itisertions in pBR322 were found to be distributed around the plasmid, but insertions in certain regions occurred more frequently than in others.