A method for the determination of nonionic surfactant (Cobalt Thiocyanate Active Substances, CTAS and polyoxyethylene alkylphenyl ethers) in environmental samples has been developed. Nonionic surfactant in a water sample was extracted in a stripping apparatus (Sublator) with ethyl acetate while passing nitrogen stream, and the clean-up procedure was carried out with a silica gel column by use of chloroform as mobile phase, and then the nonionic surfactant was eluted with methanol or with methanol-chloroform mixture (1 : 25). After having been passed through cationic and anionic ion-exchange resin column, the eluate containing the nonionic surfactant was determined by colorimetry of cobaltothiocyanate method with polyoxyethylene nonylphenyl ether (EO : 9.00) as standard substance. When this method was applied to river waters and waste waters in 15 districts throughout Japan, CTAS was detected in 26 samples of river water among 87 (the maximum content : 500μg/l) and 3 samples of waste water among 15 (1080μg/l). Furthermore, the content of CTAS was about 1/10 to that of Methylene Blue Active Substances, and about 79 percent of the content was polyoxyethylene alkylphenyl ethers.