The plugging of downflow fixed film anaerobic reactors owing to solids overloads, biomass accumulation, or both, has been identified as a potential operational problem, particularly with fixed bed processes (1). Although anaerobic biofilms have often been reported to be slimy and semi-fluid, they can often reach excessive thicknesses on a stationary medium, perhaps as a result of the hydraulic support provided by the liquid in the reactor. In an attempt to avoid this difficulty, the feasibility of a new process configuration was investigated: the anaerobic trickling filter. The characteristics of the process were similar to those of a downflow stationary fixed film (DSFF) reactor (2), with the exception that, like an aerobic trickling filter, the anaerobic version operated with the media unsubmerged.