A rapid and simple method for the determination of anionic surfactants in tap water has been developed. The official analytical method in Japan for the determination of anionic surfactants in tap water requires tedious procedures and requires large amounts of chloroform as the extract solvent; it was greatly improved by the present spectrophotometric method using Methylene Blue. Our present method requires only one half of sample (50 ml), one tenth of the extraction solvent (chloroform=5 ml), and one sixth of the analytical time compared with the official analytical method.