The start-up of a mixed system, with passively immobilized Zymomonas mobilis in a fibrous matrix and suspended flocs) has been optimized for continuous ethanol production. Reactor inoculation with a flocculent variant of Z. mobilis ZM4 allowed the incubation periods previous to the continuous feeding be suppressed The flocculent characteristics of the bacterium together with the features of the system resulted in an increase of the retained biomass as dilution rate was increased. In order to obtain a high biomass concentration in the reactor in a short time, rather than operating the system at a fixed dilution rate for long periods of time, it is proposed to increase the dilution rate stepwise with short time intervals.