Abstract: The pattern of alkaline phosphatase activity in the gastrointestinal tract of steelhead trout as revealed by Gomori-Takamatsu's Ca-CoS 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 becomes 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 p H conditions suggest that phosphatases present in the brush border, lamina propria, and nuclei have different p H optima and are active over different p H ranges. The use of inhibitors in various concentrations in the incubating medium shows that the enzyme located 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 very sensitive to lower inhibitor concentrations. Experimental evidence is presented to show that the reaction in the nuclei is due to the presence of intrinsic phosphatase. Following starvation, changes in the enzyme activity pattern are indicated in the intestinal mucosa, probable factors inducing these changes are discussed. Differential patterns of phosphatase activity in tissue sections obtained under different substrates, p H values, inhibitor concentrations, and levels of starvation strengthen the belief of \"enzyme plurality\".