Evolution of earthworm (Eisenia foetida, Savigny) biomass and changes in enzyme activities during 18 weeks of sewage sludge vermicomposting were studied. With time, hydrolase (P-glucosidase, urease, BAA-hydrolysing protease and phosphatase) and dehydrogenase (DH-ase) activities decreased as available organic compounds (water-soluble carbon) decreased. A high correlation among all enzyme activities and between each activity and the water-soluble carbon (WSC) led to the conclusion that both hydrolytic and dehydrogenase activities could be feasible indicators of the state and evolution of the organic matter. The ＇potential metabolic index＇ (DH-ase/WSC) enabled distinguishing between hydrolytic and maturation phases in a sewage sludge vermicomposting process.