A continuously stirred tank reactor (181) was used as an anaerobic reactor to examine the effect of adding fish offal to a system which was digesting cattle slurry. The retention time was kept at 21 days and the loading rate maintained in the range 5.1-6.3 kg volatile solids (VS) m(-3) d(-1). 4% (w/w) fish offal caused some deterioration in the performance of the digester, judged in terms of both the VS reduction and methane yield. 6% fish offal caused a significant disruption in the digester＇s performance. The concentrations of free ammonia which were present in the liquors tended to decrease when the fish offal was present and did not exceed 100 mgl(-1). This is below the concentrations which have previously been shown to be inhibitory. Volatile fatty acid concentrations increased during the additions of the fish offal. Taken in conjunction with the high fat/oil concentration of the fish offal, it was considered that long chain fatty acids were the most likely cause of the digester failure.