Summary: Today already numerous micromycetes are a recognized part of the biocenosis of growth in receiving waters strongly polluted by organically rich waste waters and of growth on biofilter media surfaces. The principal representatives are some Deuteromycetes, Mucorales and Saccharomycetaceae, whereas the so-called true aquatic fungi (Oomycetes) with the exception of Leptomitua lacteua are practically absent. The growth of micromycetes in activated sludge remains still an unanswered question. Generally, the filaments in the activated sludge are ascribed to filamentous bacteria and as long as fungi were isolated from activated sludge their presence has been reported only in the form of spores.The nitrogen deficiency with regard t o carbon present in the waste water is considered as one of the factors enhancing the development of fungi in the activated sludge. In a series of 13 tests carried out on laboratory-scale batch models using synthetic waste water, where the carbon source was glucose and the nitrogen source was (a) sodium nitrate, (b) ammonium sulphate, and (c) peptone, the fungal growth was studied in different models with the B0D:N ratio varying from 20 to 1000. The models were seeded with thickened activated sludge from the municipal sewage works. The sludge did not contain any microscopically detectable forms of fungoid growth. It was found that the fungi in activated-sludge medium grow well end both in the forms of single cells, occuring independently and in chains, as well a8 in the form of long branched filaments. In some instances, the micromycetes formed up t o 80 t o 100 per cent of the biocenosis of the activated-sludge flocs, but the dependency of their development on the increasing B0D:N ratios in the waste water has not been ascertained.Fifteen species of micromycetes were isolated and identified, and in addition several representatives of Saccharomycetaceae, which were not identified.