This paper presents the behaviour of selected. bacteria, the mixture of, and indigenous to municipal sewage microorganisms, in biodegradation of benzene, toluene, o-xylene and p-xylene (BTX). Apart from an expected different behaviour under aerobic and anaerobic conditions the difference in degradation rate obtained for the different microorganisms tested, suppositions on practical application of the results were made. It was found that the mixture of microorganisms present in municipal sewage can be advantageously used in biodegradation of aromatic hydrocarbons under aerobic conditions. Opposite conclusions could be drawn from the experiments performed in anaerobic conditions where selected specific bacteria have been much more active. The observations made, of toluene biosynthesis. under specific anaerobic conditions seem to be totally new and unique.