Cyanocystis violacea isolated from a marine rock sample conforms with the diagnosis ofDermocarpa violacea Crouan in all significant characteristics. The distinct heteropolarity of the cells and simultaneous cell divisions, are stable characters in culture. Development and growth of cells, simultaneous cell division and nanocyte formation have been documented by single-cell slide cultures and fine structural studies. The reddish violet color of the cytoplasm is due to the abundance of phycoerythrin.