Abstract—The rates of the photosensitized immobilization of the nauplii of the crustaceanArtemia salinawere measured as a function of irradiation time and the amount of light absorbed by the sensitizers. The nauplii were incubated in the dark in dilute solutions of the sensitizers for periods of 2 and 22 h prior to irradiation. Nineteen carcinogenic and 22 noncarcinogenic polycyclic aromatic hydrocarbons were used as sensitizers. Relative photodynamic activities (RPA) were determined from the rates of immobilization using benz[c]acridine as a standard (RPA = 1). High RPA was restricted to carcinogenic compounds with 4 and 5 fused rings, compounds with 6 or more fused rings had low RPA regardless of their carcinogenic activities. The correlation of RPA with carcinogenic activity was excellent (P= 0.006 and 0.009 for the 2 and 22 h dark incubations, respectively). It is suggested that carcinogenesis by polycyclic aromatics may result from sublethal photodynamic effect