Pretreatment of orange processing waste (OPW) by solid-state fermentation using selected strains of Sporotrichum, Aspergillus, Fusarium and Penicillium improved overall productivity of biogas and methane. Fungal pretreatment enhanced the availability of feed constituents and reduced the level of antimicrobial substances and allowed the use of a lending rate of OPW of 8-10% (dry weight) compared with untreated feed of 4%. Anaerobic digestion of pretreated feed for six months in 1500 litre KVIC (Khadi and Village Industries Commission) digesters yielded an average biogas yield of 0.5-0.6 m(3)/kg volatile solids added at a loading rate of 8% with a 25 day hydraulic retention time (HRT). Higher concentrations of pretreated OPW posed serious operational problems due to plugging of the digester The methane production by this process demonstrated the stability of the digester throughout the period of study with a volatile fatty acids (VFA) of 600-800 mg/litre (as acetate), pH 6.8-7.0 and total alkalinity of 2200-3000 mg/litre (as CaCO3).