Six hundred and twenty species and lower taxonomical units of Rotatoria found in Czechoslovakia and surrounding countries are listed alphabetically and classified according to water quality. The numerical characteristics include the saprobic valence in 10 balls, the indicative weight of species, Ii, and the individualsaprobic index, Si. Rotifers are considered to be good indicators and some of them are figured on Plates 1-3. The situation is illustrated by four graphs and the relation to BOD5 values is stressed. All rotifers are aerobic organisms and occur only within limnosaprobity. They can also serve as indicators of trophic conditions. To characterize the situation in standing and slowly flowing waters a Brachionus:Trichocerca quotient is proposed. Rotifers can also be used as test organisms in toxicity experiments.