Anaerobic digestion is a key process in the integrated three-stage system widely adopted For treating wastewater from livestock production in Taiwan. The red mud plastic (RMP) digester is the most common digester used. It is a modified baffled reactor. The current design recommendation for this digester is based on a 10 days hydraulic retention time (HRT). This seems an over-design for dairy wastewater, since such wastewater is often very dilute due to large volumes of cleaning water used. This study reports on the performances of four types of anaerobic digesters (CSTR, UASB, UAF and baffled) treating very dilute dairy wastewater. Experiments were conducted with HRTs between 18.8 and 2 days and with organic loading rates between 0.117 and 1.303 g VS l(-1) day(-1). Establishment of methanogenesis seems slower for the CSTR and the baffled reactors. The baffled reactors could achieve a solids retention time (SRT) of 31 days when its HRT was 4 days. Effluent SS and COD from the UASB, the UAF and the baffled reactors operated at HRTs of 5, 4 and 3 days mel discharge standards. The baffled reactor is preferred for its operational simplicity. The results indicated that for very dilute dairy wastewater, the design HRT of a baffled reactor could be shortened to less than 5 days. Copyright (C) 1996 Elsevier Science Ltd.