The swelling of mixed oxide in fuel elements irradiated within the irradiation program of the German Fast Breeder Project was determined. In fuel pins with a burnup-independent high power level, a constant swelling rate of 0.5% per percent burnup was found at burnups above 4%. It is concluded that this swelling rate is induced by solid fission products. The volume-averaged swelling due to fission gases seems to be saturated at a value of ∼6%.