In general, enzyme reaction rates continue to increase at higher temperatures, even though the temperatures increase beyond the lethal temperatures for the species, until the enzymes begin to denature around 50-60°C. On the other hand, enzymes have limited ranges of pH over which they function, often as little as 2 pH units. Data for channel catfish are probably representative of many teleosts. Acid concentrations (pH) in the stomach ranged from 2 to 4, then became alkaline (pH = 7-9) immediately below the pylorus, decreased slightly to a maximum of 8.6 in the upper intestine, and finally neared neutrality in the hindgut (Page et al., 1976). Fish having no stomach have no acid phase in digestion.