Cyanine dye compounds having a negatively charged substituent that are nucleic acid stains, particularly for fluorescent staining of DNA, including compounds having the formula wherein W forms one or two fused 5- or 6-membered aromatic rings, a has a value of 0 or 1, n has a value of 0, or 1, X is O, S, or Se, and D is a pyridinium, or quinolinium moiety, provided that the compound has at least one negatively charged substituent.