In this work, the effect of the feeding strategy in Zymomonas mobilis CP4 fed-batch fermentations on the final biomass and ethanol concentrations was studied. Highest glucose yields to biomass (0.018 g/g) and to ethanol (0.188 g/g) were obtained in fed-batch fermentations carried out using different feeding rates with a glucose concentration in the feed equal to 100 g/l. Lower values (0.0102 g biomass/g glucose and 0.085 g ethanol/g glucose) were obtained when glucose accumulated to levels higher than 60 g/l. On the other hand, the highest biomass (5 g/l) and ethanol (39 g/l) concentrations were obtained using a glucose concentration in the feed equal to 220 g/l and exponentially varied feeding rates. Experimental data were used to validate the mathematical model of the system. The prediction errors of the model are 0.39, 14.36 and 3.24 g/l for the biomass, glucose and ethanol concentrations, respectively. Due to the complex relationship for describing the specific growth rate, a fed-batch culture in which glucose concentration is constant would not optimize the process.