Here we studied how the transit through the gut of the earthworm Eisenia fetida affects the microbial and nutrient stabilization of pig and cow manure, by analyzing fresh casts. Earthworms reduced the pools of dissolved organic C and N in casts from both types of manure, as wells as mineral N. Microbial biomass was enhanced only in casts from pig manure and did not change in casts from cow manure, and fungal populations only raised in casts from cow manure. Earthworms reduced microbial activity in casts from cow manure and did not modify in casts from pig slurry. Enzyme activities in casts also depended on the manure ingested; there were no changes in dehydrogenase and P-glucosidase activities, whereas acid and alkaline phosphatases increased. The results indicate that the first stage in vermicomposting of pig and cow manure by E.fetida, i.e. casting, produced a microbial stabilization decreasing the activity of microorganisms; this stabilization occurred despite of the increase in microbial biomass. The strong reduction in nutrient pools of manures may be the responsible of this contradiction. These changes will influence the dynamics of the organic matter degradation by reducing forms of C and N available to microorganisms and hence restricting their growth and multiplication. Nevertheless, casts were also characterized by an increased enzyme potential that might lead to a further thorough degradation of pig and cow manure. (C) 2008 Elsevier B.V. All rights reserved.