Biodegradations of Na-lactate and Na-acetate were evaluated in microbial fuel cell (MFC) type bioreactors. Increase in lactate concentration from 1,000 to 5,000 mg L(-1) enhanced the biodegradation rate from 4.6 to 23.9 mg L(-1) h(-1). Sequential batch operation of MFC enhanced the lactate biodegradation rate. With acetate, neither increase in concentration nor sequential operation had a marked effect. Maximum power and current densities in MFCs operated batch-wise with lactate and acetate were 3.30 and 2.28 mW m(-2), and 48.2 and 40.2 mA m(-2), respectively. In the MFC operated continuously, increase in lactate loading rate caused the biodegradation rate to pass through maximum value of 1,668.2 mg L(-1) h(-1) (residence time: 1.2 h). Open circuit potential, power and current densities for continuous operation were 700 mV, 8.10 mW m(-2) and 43.0 mA m(-2), respectively. Using the experimental data, kinetic models for microbial growth and biodegradation of lactate and acetate in the MFC were developed.