This paper presents the results of nine years of experience in design and operating of full-scale anaerobic-aerobic treatment plants in a fermentation industry producing baker's yeast from sugarbeet molasses. The PAKMAYA Izmit Factory has a large two-staged treatment plant since 1986: anaerobic first stage and aerobic second stage. The anaerobic reactors were constructed as Upflow Anaerobic Sledge Blanket Reactors (UASBR) with internal and external sludge recirculation facilities. Average COD removals remain in the range of 75% in the mesophilic anaerobic stage. Average daily biogas production rates are as high as 20,000 m(3)/day. This treatment plant is one of the largest in the world in terms of biogas production. Similar systems were constructed later in two other factories of the same company (Cumaova-Bolu, Kemalpasa-Izmir). The biogas conversion yield is about 0.65 m(3) per kg COD removed. The energy produced from the biogas used in the boiler houses is about 35% of the total energy requirement of the factory. The effluents from the anaerobic first stage are fed to the subsequent aerobic treatment system by mixing with low strength effluents of the factory. The aerobic second stage was designed and operated as an extended aeration activated sludge system, The COD removals of the aerobic stage are averaging at about 60 to 75%. This paper also discusses the operating problems encountered in the various stages of the treatment system and their solutions during the nine years of full-scale operation in three different treatment plants of PAKMAYA. Copyright (C) 1996 IAWQ. Published by Elsevier Science Ltd.