S'LlUL?>L@r?J:The behaviour of the herbicide bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), labelled with 82Br, is investigated in a n activated sludge plant on the laboratory scale with municipal wastewater. Only 5 o/o of bromoxynil present in a concentration of 5 mgil are degraded after preadaption of activated sludge for some weeks. The known hydrolysis products 3,5-dibromo-4-hydroxybenzamide and 3,5-dibromo-4-hydroxybenzoic acid were identified as degradation products. I n addition, 1.9$0.9 O/o of the quantity of R2Br appeared in the solution as Br-by debromination. Moreover, a pronounced adsorption of the active substance on activated sludge occurs. I t s activity amounts to about 130 o ! ,, of the initial activity of the solution.