The 24- and 48-hr EC50 immobilization test of Daphnia magna has been performed on 14 herbicidal triazines characterized by structural differences with respect to the substituent groups at the three carbon atoms in the s-triazinic ring. EC50 values of the tested compounds were found to vary within a factor of 6 to 7; such variations of toxicity have been analyzed in terms of structural variations concerning (i) the type of substituents, i.e., Cl, SCH3, or OCH3, at the carbon atom C-2, and (ii) the number and size of the alkyl groups at the aminic nitrogen atoms bound to the ring carbons C-4 and C-6. Literature data relative to the acute toxicity of the considered herbicides on fish and plants were also analyzed with reference to the above structural features. The possibility that separate contributions to toxic effects of triazines are provided by specific groups bound to the common structural moiety, constituted by the 1,3,5-triazinic ring, is examined and discussed.