Water balances, based on the evaluation of water released from the fermentor, were estimated in a solid-state fermentation process performed without forced aeration. It was proved that the net gain or loss of water during the course of the process depends on the dry weight of the solid substrate at the beginning of the process and also on the initial water activity. A linear relationship was found between the wet weight losses of cultures recorded after the end of the growth period and the initial dry weight of the substrate which enables the evaluation of the water released from om the vessels. These values were included into water balances from which it was possible to calculate the water produced by metabolism of carbohydrates, which was estimated at 0.40 mol of H2O per C-mol of glucose consumed.