The evolution of water activity during solid-state cultivation of Gibberella fujikuroi was followed. A typical organic substrate, wheat bran and soluble starch, was used. Culture sorption isotherms were determined verifying that, as culture evolves, higher moisture contents were necessary to maintain the same water activity level. Optimal values for Gibberella fujikuroi growth and gibberellic acid production rates and yields were established, around a(w) 0.99. A non-linear model, based on neural networks, is proposed to represent the sorption curves of the substrate during the fermentation process. (c) 2005 Elsevier Ltd. All rights reserved.