Novel high rate psychrophilic (20 degrees C) anaerobic digestion (PAD) of undiluted cow feces (11.5-13.5% total solids) was demonstrated using sequence batch reactor in long-term operation with successive cycles of 21 days treatment cycle length (TCL). At organic loading rates (OLR) 9.0, 10.0, 11.0 and 12.0 g TCOD kg(-1) inoculum d(-1) average specific methane yield (SMY) was 154.0 +/- 11.7, 152.1 +/- 12.2, 126.0 +/- 2.8 and 116.0 +/- 6.1 NL CH4 per kg of VS fed, respectively. Volatile solids removal averaged around 31.7 +/- 3.3%, 32.2 +/- 1.0%, 27.9 +/- 2.2% and 23.4 +/- 0.5%, respectively. Substrate-to-inoculum ratio (SIR; wet-mass basis) ranged between 1.17 +/- 0.06 and 1.43 +/- 0.05. Concentration of volatile fatty acids in the bioreactors during the TCL indicated that hydrolysis was the rate limiting reaction. High rate PAD of undiluted cow feces is possible at OLR (g TCOD kg(-1) inoculum d(-1)) 9.0 and 10.0 with a TCL of 21 days; however, OLR of 11.0 and 12.0 are also possible but require longer TCL to maintain the SMY. Crown Copyright (C) 2015 Published by Elsevier Ltd. All rights reserved.