Bio leaching is becoming increasingly used for the extraction of gold from refractory gold ore. It is established that in gold bio leaching two kinds of mechanism are involved namely indirect bio leaching and direct bio leaching. The reaction schemes involved in the different mechanisms and kinetic rate expressions for different reactions have been reported in the literature. In the present paper an attempt has been made to develop an approach for simulation of a gold bio reactor leach circuit. For this purpose a mathematical model for bio leaching has been developed for process design of a gold bio reactor circuit. The salient features of the model are parameter estimation from experimental data and then scaling up the process for pilot size and industrial size bio reactor operation in continuous mode. Bio reactors can be simulated for various combinations of series-parallel modes of operation. Experimental data of the laboratory scale bio reactor collected for different pulp densities and residence times have been simulated for parameter estimation. Experimental data of pilot size bio reactors, collected from the literature, have been used for vetting of the parameters estimated earlier. Analysis of the simulation results indicates the extent of conversion taking place due to indirect and direct mechanisms. The performance of the simulation model developed based on literature information has been confirmed by some experimental data collected as well as by literature data. (C) 2001 Elsevier Science Ltd. All rights reserved.