SUMMARYCertain types ofActinomycesproduce a pungent odour which has frequently been described as “earthy”. This odour is caused by an organic compound which is slightly soluble in water, volatile in steam, soluble in ether, and partly soluble in alcohol. In concentrated form the substance forms a brown amorphous material with a penetrating manurial odour.When dissolved in water in concentrations as low as two parts in ten million it imparts to the water a pungentActinomycesor “earthy” taint which is particularly noticeable when the reaction of the water is maintained on the alkaline side of the neutral point.The existence of an excessive contamination of an important salmon river with odour‐producing actinomycetes was established. This contamination and the resultant pollution of the water with a pungent “earthy” taint was restricted to the tidal part of the river, where decaying organic matter was abundant.Acknowledgment is due to the Inspector of Salmon Fisheries for Scotland for his interest in the work. Thanks are also due to the Director of Chemical Research in the Department of Scientific and Industrial Research for facilities afforded in publication of the results of these experiments with which the Water Pollution Research Board of the Department has also b