Alkaline phosphatase, ALPase (E.C. 3.1.3.1) was immobilized onBombyx morisilk fibroin fiber by covalent bond formation. Two different immobilization procedures (i.e. diazo and cyanogen bromide, CNBr, methods) were tried. The immobilization conditions such as pH, enzyme concentration, reaction time, and temperature were examined in detail, and optimum conditions were obtained. The immobilized enzyme activity was characterized with Michaelis constant,Km, and maximum activity,Vm.The optimum pH of the activity of the fixed enzyme tended to shift to the acid side. Thermal stabilities of the enzyme were improved above 50°C. In addition, the immobilized enzyme maintains activity over a long period.