Gracilaria chilensis were batch fermented with different initial carbohydrate concentrations and inoculated with an initial fixed optical density of the yeast Saccharomyces cerevisiae. To determine the kinetics and effects on the kinetic parameters of the bioethanol production process, the fermentations were performed under controlled oxygen, pH and temperature conditions. When high carbohydrate concentrations were used as substrate, a rapid decline in their concentration was observed, but the development of the yeast's metabolic activity was very slow. Besides, the saturation constants (Ks) were high and the maximum specific growth rate (mu(max)) of the yeast remained within the same order of magnitude, without significant variation, although the ethanol yield was low. When an initial low substrate concentration was used, an increase in ethanol production (Yp/s) and in cellular growth (Yx/s) was observed. The positive impact on the development of the microorganism's metabolic activity was evidenced by a decrease in mu(max) from 0.47 to 0.29h(-1) and in Ks from 15.19 to 1.07g.l(-1). The development of the microorganism's metabolic activity is influenced by the carbohydrate concentration of G. chilensis when other physicochemical parameters of the fermentation process are kept constant.