The performance of an anaerobic downflow stationary fixed film bioreactor has been simulated mathematically and a software package developed. Each flow channel of the reactor is assumed equivalent to a tubular reactor with a specific degree of axial dispersion. The variation of axial dispersion coefficient along the length of the flow channel (due to the change in gas velocity ) is accounted for and so is resistance to substrate transfer into biofilm which is taken into account by incorporating effectiveness factor into performance equations. Effectiveness factor is computed by numerically integrating the expression for intrinsic rate and the package is applicable to all forms of kinetic equations. Experimental data from pilot plants exhibit encouraging agreement with results predicted by the developed simulation package.