The effects of mild organic pollution on the invertebrate fauna of the Great Berg River in the Paarl-Wellington region are described and also the conditions in a polluted tributary, the Dwars River. Serious organic pollution, largely from distillery wastes, was encountered in the Krom Stream, Stellenbosch; the drastic changes in the composition of the bottom fauna are described and related to changes in water quality. There is a discussion on the effects of different degrees of organic pollution on the faunal associations of various biotopes and reasons are advanced for the changes encountered, ‘indicator species’ are also discussed. Conditions found in South Africa so far are compared with those reported from other parts.