The interactions of nile blue sulphate (NBS) with nucleic acids, including calf thymus DNA, fish sperm DNA and yeast RNA, were characterized with resonance light-scattering (RLS) measurements by using a common spectrofluorometer. Accordingly a method for the determination of nucleic acids at nanogram levels was established. At pH's of 7.20∼7.60 and ionic strengths lower than 0.012, the interactions of NBS with nucleic acids result in three characteristic RLS peaks at 293.4 nm, 349.4 nm and 560.4 nm. Mechanism study shows that these peaks are ascribed to the long range assembly of NBS on the molecular surface of nucleic acids, which depends on pH, ionic strength and the stranded structure of nucleic acids. A Scatchard plot was constructed by using the RLS data, yielding the assembly number and assembly constant being 6.4 and 7.13x106mol−11 for NBS assembly on the molecular surface of calf thymus DNA. The same parameters are 6.6 and 4.58x106mol−11 for the assembly on that of fish sperm DNA, 3.9 and 1.67x106mol−11 on that of yeast RNA, respectively. Linear relationships were found between the enhanced RLS intensity at 293.4 nm and nucleic acid concentration. If 1.2x10−5mol I−1NBS was employed, 0∼0.80 μg ml−1calf thymus DNA and fish sperm DNA, 0.20∼0.60 μg ml−1yeast RNA can be determined with the determination limits being 3.2 ng ml−1for calf thymus DNA, 11.5 ng ml−1for fish sperm DNA and 38.3 ng ml−1for yeast RNA, respectively. Four synthetic samples were determined with satisfaction.