Liquid-solid mass transfer in a two phase liquid-solid anoxic fluidized bed bioreactor used for biological denitrification has been studied. The denitrification microorganism, immobilized on plastic beads was used as fluidization media. The denitrification rate under different operating conditions was studied and was found to follow zero order kinetics with respect to substrate. The results of experimental work were used to simulate a biofilm model for the evaluation of liquid-solid mass transfer coefficient using the value of concentration obtained at the biofilm surface. The effect of parameters like liquid flow rate and biofilm thickness on mass transfer coefficient were studied. A functional relationship relating Sherwood number to Reynolds and Schmidt number is obtained by regression analysis as Sh=Re-0.5985 Sc-0.2048. (c) 2007 Elsevier B.V. All rights reserved.