This study evaluated the potential of earthworms (Eisenia fetida) to remove pathogens from the sludge from septic tanks. Three earthworm population densities, equivalent to 1, 2, and 2.5 kg m(-2), were tested for pathogen removal from sludge. The experimental phase lasted 60 days, starting from the initial earthworm inoculation. After 60 days, it was found that earthworms reduced concentrations of fecal coliforms, Salmonella spp., and helminth ova to permissible levels (<1000 MPN/g, <3 MPN/g, and <1 viable ova/g on a dry weight basis, respectively) in accordance with Official Mexican Standard of environmental protection (NOM-004-SEMARNAT-2002) (SEMARNAT, 2002). Thus, sludge treatment with earthworms generated Class A biosolids, useful for forest, agricultural, and soil improvement. (c) 2009 Elsevier Ltd. All rights reserved.