Mixed bacterial communities from the rumen ferment cellulosic biomass primarily to C-2-C-4 volatile fatty acids, and perform only limited chain extension to produce C-5 (valeric) and C-6 (caproic) acids. The aim of this study was to increase production of caproate and valerate in short-term in vitro incubations. Coculture of mixed ruminal microbes with a rumen-derived strain of the bacterium Clostridium kluyveri converted cellulosic biomass (alfalfa stems or switchgrass herbage) plus ethanol to VFA mixtures that include valeric and caproic acids as the major fermentation products over a 48-72 h run time. Concentrations of caproate reached 6.1 g L-1, similar to or greater than those reported in most conventional carboxylate fermentations that employ substantially longer run times. Published by Elsevier Ltd.