A rotifer mono-species-test is proposed for the assessment of sub-lethal effects to this freshwater organism group. The qualification of the test method was examined in practice. The test species selected were Brachionus angularis and B. rubens; lindane served as a model chemical. A semi-automatic counter for the frequent quantification of high population densities was constructed. As indicator for sub-acute toxic effects of lindane the deviations in the population development to untreated controls were investigated. For both test species NOECs were established after 28 days lindane treatment.