Are plants generally less sensitive to toxicity than aquatic animals as indicated by some authors (1)? The use of phytotoxicity (higher plant) tests as a part of ecotoxicology is relatively underdeveloped. The objective of this study was to compare the results of the OECD Lemna Growth Inhibition Test with other mainly used test systems. Three toxicants were tested applying the Alga Growth Inhibition Test (Seleneastrum capricornutum), the Daphnia Immobilization Test (Daphnia magna) and the MICROTOX-test (Photobacterium phosphoreum). Duckweed plants were found to be sensitive to the chemicals tested, with nearly 100% inhibition of frond production occurring in the highest concentrations. The plants in some samples showed lesions (break up of the colony structure) and loss of green pigments (chlorosis), others showed localized dead tissues (necrosis). In addition to the morphological deviations (appearance) there is a wide range of characteristics to be observed on the test plants: number of fronds, number of plants, biomass, dry weight, mortality, CO2 and O2 exchange, concentration of chlorophyll a and chlorophyll b. It is consistent that the number of fronds is slightly more sensitive than or the same as the number of plants and that the biomass tend to be somewhat more tolerant than those for the number of plants (2). Applying the simplest way of observation, changes in the frond number were recorded. As Lemna minor show a characteristical sensitivity to the different kind of chemicals, it is a promising indicator of aquatic toxicity.