When fed at the level of 50 P;; of the diet, the dried residue of Ascophyllum nodosum, remaining after alginate extraction, was found to cause diarrhoea in pigs within 7 days. Digestibility coefficients for a diet containing 50% seaweed residue, 20% barley, 19 % wheat and 10% fish meal were 0.47 for energy and 0.52 for nitrogen. By difference calculation the digestible energy and digestible crude protein values of the seaweed residue were determined to be 2.2 MJ and -30 g/kg dry matter.