AbstractIn continuation of a publication on the behaviour of fluorotensides in aqueous solution there were tested four selected fluorotensides, among them a noniononic ethoxylated substance and three anion‐active substances, in two different equipment with respect to their biochemical degradability. The BOD5, the potassium permanganate consumption, the dry matter contents and the loss on ignition as well as the dehydrogenase test were used as parameters for judging the degradation behaviour. In all the four cases a high biochemical stability was found, so that they must be classified into the group C “biochemically not degradable or difficult to degrade” in agreement with the results found, according to the water pollutants cata