Fed-batch fermenters are generally operated with the addition of small doses of nutrients, therefore the volume of the fermentation broth increases with time. Batch fermenters generally contain and almost constant volume of broth and a logistic equation has been commonly employed to simulate microbial growth in them. Mass balances were determined with fed-batch fermentation to obtain expressions which account for the effect of volume increase and the subsequent dilution of the biomass. A growth rate expression was obtained for fed-batch fermentations. The new expression was very similar to the logistic equation. Therefore an anology between the values of the parameters of the new model and of the logistic equation was sought. The new model was also employed to simulate six sets of data from the literature and satisfactory results were obtained.