ATTO 550 is a novel fluorescent label related to the well-known dyes Rhodamine 6G and Rhodamine B. Characteristic features of the label are strong absorption, high fluorescence quantum yield, and high thermal and photo-stability. The dye is moderately hydrophilic. ATTO 550 is a cationic dye. After coupling to a substrate the dye carries a net electrical charge of +1. As supplied ATTO 550 consists of three isomers with practically identical absorption and fluorescence. The fluorescence is excited most efficiently in the range 540 - 565 nm.