Irradiation of the photoheterotrophic cyanobacterium Synechocystis sp. PCC 6803 with low levels of UV light allows for stable, integrative transformation of these cells by heterologous DNA. In this system, transformation does not rely on an autonomously replicating plasmid and is independent of homologous recombination. Cells treated with UV light in the absence of DNA and cells given DNA but not exposed to UV do not yield antibiotic-resistant colonies in platings of up to 2 X 10(8) cells. Optimal conditions for this UV-induced transformation are described. Analysis of the transformants indicates that (i) only a segment of the introduced plasmid is found in the DNA of the transformed cells; (ii) in independently isolated clones, DNA insertion apparently occurs at different sites in the chromosome; and (iii) hybridization data suggest that insertion in one of the transformants may have occurred into a region of the chromosome that is repeated or that integration of plasmid DNA may have been accompanied by a rearrangement or duplication of DNA sequences near the insertion site. DNA isolated from the primary transformants as well as a cloned fragment containing the UV-inserted plasmid sequence and flanking cyanobacterial DNA transform wild-type cells at a high frequency (5.0 X 10(-4) and 1.5 X 10(-5), respectively). Possible mechanisms of this transformation system are discussed, as are the potential uses of this system as an integrative cloning-complementation vector and as a mutagenic agent in which the genetic lesion is already tagged with a selectable marker.