A simple structured model is developed for the description of the experiments of continuous production of a hybrid antibiotic by Streptomyces lividans TK21 pellets in a fluidized-bed reactor. The model is based on the effect of internal and external phosphate concentrations on antibiotic production during cyclic feeding. These concentrations can be calculated on the basis of the equations postulated by the model. The model also considers the cell growth, reflected in changes of the pellet size along the culture. The model parameters are evaluated sequentially by performing experiments at different operational conditions. The validity of the model and its corresponding parameters is corroborated further by the satisfactory modeling of the bioreactor operation during an extended period of time at various operation conditions.