Abstract A method of fuzzy modeling for a microorganism reaction is proposed. The structure of this model is based on the conventional logistic model. The consequents of the production rules of this model are made up of linear functions in which a logistic model is incorporated. The parameters of our fuzzy model were identified by the simplex method. We applied this model to describing the batch culture process of Zymomonas mobilis, which had been studied by us in earlier research. This process can be explained and estimated. Using this construction method, a simple fuzzy model can be constructed and compared with a conventional deterministic (logistic) model.