The pattern of alkaline phosphatase activity in the gastro-intestinal tract of steelhead trout as revealed by Gormori-Takamatsu’s cobalt sulphide method indicates that this enzyme is intimately concerned with differentiation. During the early stages of development, the enzyme is widely distributed and localized mainly in the cytoplasm and nuclei of the gut primordium, later on, however, it tends to become electively localized in those areas where active differentiation is taking place, and disappears from other regions. Regional accumulation and loss of alkaline phosphatase throughout the period of gut differentiation points to the characteristic discontinuity in enzyme development. In the fully developed gut, phosphatase activity is localized in the gastric and intestinal regions in the lamina propria, brush border, and nuclei of the tunica mucosa. The probable role of phosphatase in these locations is discussed. Topographically variable patterns of enzyme activity obtained under different pH conditions suggest that phosphatases present in the brush border, lamina propria, and nuclei have different pH optima and are active over different pH ranges. The use of inhibitors in various concentrations in the incubating medium shows that the phosphatase present in the brush border is more resistant to high concentrations of inhibitors than that in the lamina propria. Nuclear phosphatase, on the other hand, is remarkably sensitive to lower inhibitor concentrations. Under moderate starvation, an increase in the enzyme activity is indicated in the brush border of the intestinal mucosa; acute starvation brings about structural deterioration of the mucosa and desquamation of the columnar epithelial cells, leading to diffuse phosphatase reaction. Probable factors inducing phosphatase accumulation following starvation are discussed. Differential patterns of phosphatase activity in tissue sections obtained under different substrates, pH values, inhibitor concentrations, and levels of starvation strengthen the belief of 'enzyme plurality'. The nuclear and extranuclear phosphatases behave differently and probably are not identical. / Science, Faculty of / Zoology, Department of / Graduate