The interaction effects of five variable parameters on gallic acid yield by the application of Evolutionary Operation (EVOP)factorial design technique is presented in this paper. The methodology combines the advantage of factorial techniques for designing experiments with five parameters and that of EVOP methodology for the systematic analysis of experimental results. It facilitates the selection of optimum conditions or directs the changes desired for individual parameters for the designing of subsequent experiments. Decision were taken according to the decision-making procedure available in the literature. Standard deviation and error limits based on 95% confidence were also calculated according to the relationship given in literature. The best combinations of the physicochemical parameters at optimal levels of 32 degreesC, 93% relative humidity, 4.5 pH, 72 It incubation period, 20 g substrate weight and 0.4:1 solid-liquid ratio resulted in gallic acid yield of 90.9% under modified solid-state fermentation (MSSF). (C) 2002 Elsevier Science Ltd. All rights reserved.