Abstract1. In many countries of the tropical and sub‐tropical regions, the water hyazinth has become a priority weed problem. The damage effects of the water hyazinth are very versatile and, therefore, control has proved to be absolutely necessary. This control may be realized mechanically, biologically and chemically, with chemical control being the most effective one so far.2. The basis must be the determination of those quantities and concentrations of the active substances that will enable us to obtain the optimum herbicide efficiency in the control of water hyazinths with the minimum disturbance of the ecosystem.3. Differences in the herbicide effect in the control of water hyazinths resulted from the different application procedures. There, the best possible effect of control was obtained by means of the spraying procedure having the greatest importance also in practical application. Due to their acute toxicity, the herbicides Gramoxone, Reglone and Azaplant‐Kombi proved to be especially favourable. Azaplant had a lower and slower initial effect which could be improved in combination with Sys 67 Omnidel. In order to obtain an optimum phytotoxic effect with the simultaneously least disturbance of the ecosystem, converted 31 Gramoxone, 3 1 Reglone, 25 kg Azaplant‐Kombi, 30 kg Azaplant and 20 kg Azaplant in combination with 25 kg Sys 67 Omnidel per hectare water surface proved to be favourable.4. In the application via the water body, satisfying results could be obtained by means of Azaplant with an optimum concentration of 3.8 ppm and in combination with Sys 67 Omnidel (3.8 ppm + 6.7 ppm Sys 67 Omnidel). Application via the water body should only be realized where the spraying procedure cannot be used.5. Compared with the necessary herbicide quantities established in literature for the control of aquatic plants, the optimum values determined here are in the range of minimum quantities and they thus result in the least indispensable contamination of the w