Six insertion mutations in the gal operon of E. coli and two insertion mutations in the xycIIOP operon of bacteriophage lambda were tested for homology by annealing separated strands of lambda dgal DNA carrying the insertions, and inspection in the electron microscope. Class 1, consisting of the gal mutations OP 128, OP 141, T-N 116, OP 306, T-N 102 and the lambda mutation r14 are about 800 nucleotide pairs long, completely homologous and not circularly permuted. The first three insertions of class 1 are integrated in one direction with respect to the adjacent genes, the other three in the opposite direction. The DNA inserted in this class of mutations is called IS1. Class 2 consists of the gal insertion OP 308 and the lambda insertion r32. They are about 1400 nucleotide pairs long. The two are integrated in opposite direction with respect to the chromosome of λdgal. The DNA in insertion mutations of class 2 will be called IS 2. IS1 and IS2 do not share any detectable homology. These data are supported by cross-hybridization experiments using RNA transcribed in vitro from lambda dgal or lambda DNA carrying one insertion and DNA carrying either the same or a different insertion. Similar results were obtained by Malamy, Fiandt, Szybalski and Fiandt, Szybalski, Malamy (accompanying papers).