A method for producing gallic acid by microbiological hydrolysis of the tannins of myrobalan seed powder is described in the present work. Hydrolysis of gallotannins of the substrate to gallic acid by Aspergillus niger MTCC 282 was studied. A simple extraction procedure is used. Fungal mycelia pre-induced with 5 g/L gallotannin was used as inoculums. Optimal conditions of production were determined using various parameters including gallotannin concentration, nutritional source and metal ions are determined. Gallotannin is hydrolyzed with acid, and gallic acid in the hydrolyses is then assayed using rhodanine. This method is very specific: no interferences from other plant phenolics, including ellagic acid and condensed tannin, have been observed. The yield of gallic acid with respect to gallotannins present in the substrate is estimated. Yields of gallic acid are about 74% with respect to gallotannin concentration, which suggests that this method is exploitable industrially for the manufacturing trimethoprim drug.