The physiological activity of eleven 9-amine-9-fluorenephosphonic acid derivatives, synthesized at the Wrocław Polytechnic, was examined. The test plant was Spirodela oligorrhiza. The effect of these compounds on the increase of the dry matter of this plant was tested in eight-day experiments. The activity of the compounds was varied. The most toxic were nos. 2, 4, 9, 8, 5 and 6 which were lethal in low concentrations. Somewhat less toxic were nos. 7, 10 and 11; nos. 1 and 3 were the least toxic.