<P><B>Abstract</B></P> <P>The effects of biological co-pretreatment on biogas production from anaerobic co-digestion of food waste (FW) and waste activated sludge (WAS) were investigated. FW and WAS underwent anaerobic co-pretreatment to improve hydrolysis efficiency followed by co-digestion. Results showed that the methane yield from co-digestion of co-pretreated FW and WAS was 24.6% higher than that of control substrates without pretreatment. An increase of 10.1% in solids reduction was achieved in the anaerobic digester under the optimum pretreatment time of 24 h. Compared to mono-digestion of FW with pretreatment, the combination of anaerobic co-digestion and co-pretreatment of FW and WAS resulted in a higher treatment performance and methane production due to synergistic effects of pH amendment and enhancement of WAS particles solubilization. Pyrosequencing analysis indicated that anaerobic co-pretreatment reduced the abundance of filamentous bacteria of genus <I>Levilinea</I> in the subsequent co-digestion, which was liable to cause a decrease in anaerobic digestion performance.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A novel biological co-pretreatment method was developed for enhancing anaerobic co-digestion. </LI> <LI> Hydrolysis and acidogenesis of food waste and waste activated sludge was enhanced simultaneously. </LI> <LI> CH<SUB>4</SUB> yields increased 25% by anaerobic co-pretreatment of food waste and waste activated sludge. </LI> <LI> 10.1% more solid reduction was achieved in anaerobic co-digestion by co-pretreatment. </LI> </UL> </P>