The start-up procedure of a laboratory fluidized-bed was monitored by an automatic control system. Twenty six days were necessary to increase the load from 1 to 35 kg COD/m3.d. The system took into account the pH of the liquid phase, the production of gas, and the concentration of hydrogen in the gas phase. These parameters were measured by on-line sensors. According to variations of these parameters, algorithm based on a principle very close to an expert system adjusted the flow-rate of the feeding pump. The automatic control system was tested in the following situations: perturbations in the process generated by large pH variations (± 0.2 to 0.7 pH unit) and organic overloads (from 50 to 100 kg COD/m3.d). After 44 hours of overloading, CH4 and CO2 concentrations in the gas phase were very useful to detect the deviation of the reactor.