Prolonged operation campaigns in a fluidized bed reactor for producing granular polysilicon by deposition of silicon onto silicon seed particles from a silicon-containing precursor gas is made possible by employing a silicon-coated reaction tube which is not insulated above a region of the fluidized bed and as a result has a lower temperature such that the ratio of the thickness of the silicon on the reactor tube adjoining the fluidized bed to the coating thickness over the total reactor tube is from 7:1 to 1.5 to 1 after production campaign of from 15 to 500 days.