The Free-Wilson QSAR model in the Fujita-Ban modification was used to calculate the quantitative substituent activity contributions to the toxicity against Euglena gracilis in a series of 91 benzothiazolium salts. An important increase of activity was found for R1 = styryl or SCH2C6H5 group and for R2 = propargyl or allyl group. The substituents on benzene ring also enhance the activity. The possible relations between structure of active groups and their electronic or physicochemical properties are discussed. By using the calculated values it is possible to predict toxicity for 1300 compounds. New compounds with assumed high or low toxicity are predicted.