This review outlines microbial degradation conditions of polycyclic aromatic hydrocarbons in aquatic sediments. Microorganisms able to decompose these compounds are discussed. Concentrations of four polycyclic aromatic hydrocarbons in various sediments are presented. Bacteria and fungi decompose PAH's in different ways. Eucariota use monooxygenase, cytochrom P-450 and epoxide hydrolase to produce trans-dihydrodiols. Biodegradation of PAH's by procariota is due to dioxygenase activity. Bacteria transform these compounds to cis-dihydrodiols and then to more polar compounds with fewer aromatic rings in the molecule.