Digestion is a progressive process, beginning in the stomach and possibly not ending until food leaves the rectum as faeces. Most studies of digestion simply compare the protein, lipid and carbohydrate content of the faeces with that of the feed. A study on digestion in channel catfish by Smith and Lovell (1973) showed continuing digestion (and absorption) of protein during passage through each part of the gut (Table 1). The methods employed in this study are discussed in Section 4 below. The