It is demonstrated that PET, which is usually regarded as non-biodegradable, can effectively be depolymerized by a hydrolase from the actinomycete Thermobifida fusca. Erosion rates of 8 to 17 mu m per week were obtained upon incubation at 55 degrees C. Lipases from Pseudomonas sp. and Candida antarctica did not degrade PET under comparable conditions. The influences of crystallinity, melting point, and glass transition temperature on the enzymatic attack on PET, PBT, and PHB are discussed.