Based on the enhancement of Rayleigh light scattering of rhodamine B by nucleic acids under acidic condition at 378nm, a new method for determination of nucleic acids in aqueous solutions has been developed. This assay has a wide linear range for calf thymus DNA (0.10–16.0μgml −1 ). Besides its high sensitivity, it has some other advantages: rapidity of reaction (<2min), simplicity of operation (one-step assay), commonality of spectrofluorimeter and reagents, stability of complexes formed and reproducibility. Under the experimental conditions, there is little or no interference from proteins, nucleosides and most metal ions. Interference by some metal ions, surfactants and salts can be minimized by dilution.