A mixture of rice straw (80%) and chicken manure (20%) was pretreated and fermented to carboxylic acids by using a mixed culture of marine mesophilic microorganisms. Two sets of four fermentors, built from PVC pipes, were used for both biomass pretreatment and fermentation. Four 1 L fermentors (F1-F4) were arranged in series, where liquid fermentation products were transferred from one fermentor to the other, to form a train. A liquid volume of 10 mL and 15 mL were transferred every four days for Trains A and B, respectively. The maximum total acid concentration for F1 in Train A was 34.2 g/L and the maximum acid concentration in F2-F4 was similar to 44 g/ L. The maximum total acid concentration in F1 in Train B was 30.5 g/L and the maximum acid concentration in F2-F4 was similar to 48 g/L. The conversion in each of the fermentors in Train A varied from 0.821 to 0.879 g VS digested/g VS fed and the yield was in the range 0.489-0.609 g total acids/g VS fed. The conversion and yield in Train B were 0.741-0.914 g VS digested/g VS fed and 0.563-0.669 g total acids/g VS fed, respectively. The continuum particle distribution model (CPDM) predicted acid concentrations and retention times in the fixed-bed fermentation system with R-2 of 0.67-0.84 in Trains A and B. (c) 2006 Elsevier Ltd. All rights reserved.