Bioconversion of solid paper-pulp mill sludges and primary sewage sludge for 40 days at a ratio of 3:1 dw:dw was studied in containers with and without earthworms (Eisenia andrei). This mixture was a suitable medium for optimum growth and reproduction of the earthworms. Regardless of the presence of earthworms, degradation occurred during the bioconversion period, but the presence of earthworms accelerated the mineralization of organic matter favored the breakdown of structural polysaccharides and increased the humification rate. Consequently, the C/N ratio and the degree of extractability of heavy metals were lower in the worm-worked end product. Copyright (C) 1996 Elsevier Science Ltd.