The potential use of ram horn hydrolysate (RHH) as a supplement for improvement of citric acid production by Aspergillus niger NRRL 330 was studied. For this purpose, first RHH was produced. Ram horns were hydrolyzed by treating with acid (6 N-H2SO4) and the RHH was obtained. With the addition of RHH to the fermentation medium with a final concentration of 4% (optimal concentration), citric acid value reached a maximum value (94 g/l), which is 52% higher than that of the control experiment. The addition of 4% (v/v) RHH enhanced citric acid accumulation, reduced residual sugar concentration and stimulated mycelial growth. Adding 4% RHH had no adverse effects on A. niger. As a result, RHH was found to be suitable as a valuable supplement for citric acid production in the submerged fermentation. (C) 2003 Elsevier Ltd. All rights reserved.