This Test Guideline is designed to assess the toxicity of substances to freshwater aquatic plants of the genus Lemna (duckweed). Exponentially growing plant cultures of the genus Lemna (Lemna gibba and Lemna minor usually) are allowed to grow as monocultures in, at least, five concentrations of the test substance over a period of seven days. The objective of the test is to quantify substance-related effects on vegetative growth over this period based on assessments of selected measurement variables. The limit test corresponds to one dose level of 100 mg/L. This study includes measurement of pH, light intensity, concentrations of the test substance, the counting of the frond number and measurement of at least one other variable (total frond area, dry weight or fresh weight). From the average specific growth rates (or yield) recorded in a series of test solutions, the concentration bringing about a specified x % inhibition of growth rate (or yield) is determined and expressed as the ErCx (or EyCx). An additional response variable used is yield. In addition, the lowest observed effect concentration (LOEC) and the no observed effect concentration (NOEC) may be statistically determined.