The simple connectivity index 1X gives a reasonable account of the variation of potency with molecular structure for each of the cases presented here. The behavior of the alcohols is described better by the simple linear relationship than is the behavior of the xanthates, especially for inhibition of Tubifex worms and fish narcosis. Although a bilinear or nonlinear relationship may give improved description in these cases, lack of data in the region of higher monologs makes such equations difficult to justify on statistical grounds. The fact that the simple (nonvalence) index gives good correlation suggests that the heteroatoms (alcohol or xanthate group) plays a constant role in the biological processes. Variation in potency arises from variation in alky group structure.