A mathematical model for simulating an innovative design of a solar-heated anaerobic digester has been developed. A swine manure digester having a useful volume of 45 m(3) is constructed below ground level and its fix cover is made of Rat-plate solar collectors, which are an integral part of the roof structure. The solar collectors are coupled to a heat exchanger immersed in the digester manure. The upper part of the digester, under the tilted cover, forms an airtight enclosure that is used to collect and store the daily produced biogas. The performance of the system is investigated experimentally and the results indicate that the use of solar collectors as a cover reduces the digester thermal losses and positively affects the heat balance of the digester. The system is instrumented and an automatic data logging system is used to provide data for the validation of the system. The proposed model can accurately predict the thermal behaviour of the solar-heated digester compared to the measured data. (C) 2001 Elsevier Science Ltd. All rights reserved.