In this work, the biodegradability of wastewater from a slaughterhouse located in Kesan, Turkey, was studied under aerobic and anaerobic conditions. A very high total COD content of 7230 mg dm(3) was found, due to an inefficient blood recovery system. Low BOD5/COD ratio, high organic nitrogen and soluble COD contents, were in accordance with a high blood content. A respirometry test for COD fractionation showed a very low readily biodegradable fraction (S-S) of 2%, a rapidly hydrolysable fraction (S-H) of 51%, a slowly hydrolysable fraction (X-S) of 33% and an inert fraction of 6%. Kinetic analysis revealed that hydrolysis rates were much slower than these of domestic sewage. The results underlined the need for an anaerobic stage prior to aerobic treatment. Tests with an anaerobic batch reactor indicated efficient COD degradation, up to around 80% removal. Further anaerobic degradation of the remaining COD was much slower and resulted in the build up of inert COD compounds generated as part of the metabolic activities in the anaerobic reactor. Accordingly, it is suggested that an appropriate combination of anaerobic and aerobic reactors would have to limit anaerobic degradation to around 80% of the tCOD and an effluent concentration above 1000 mg dm(-3), for the optimum operation of the following aerobic stage. (C) 2003 Society of Chemical Industry.