The quantity of microorganisms biodegradants in samples activated sludge from municipal wastewater treatment plant at "Kremikovtzi" Holding, Sofia region, during different seasons was investigated. The total number of microorganisms was greatest during the summer. The representatives of genus Pseudomonas predominated in quantitative aspect among the bacteria. Moulds were found during all seasons, regardless of their small quantity in the total number of microorganisms. Actinomycetes were isolated only during the summer months and yeasts were not found. The performed investigations give a possibility pure cultures to be used for recovering of the disrupted ratio of microorganisms in the activated sludge at extreme situations.