Uptake of DNA has been examined in the naturally competent cyanobacterium, Synechocystis sp. Pasteur Culture Collection (PCC) 6803. This bacterium is easily transformed by exogenously offered, homologous as well as heterologous DNA. During uptake, the DNA is converted from double stranded to single stranded form. A Ca-dependent nuclease, located in the cytoplasmic membrane, probably processes the DNA offered for uptake and converts it into single stranded form. The cyanobacterial DNA-uptake process is discussed in comparison to respective phenomena in other bacteria.