Abstract: A silk fibroin membrane containing an entrapped biocatalyst such as an enzyme or microorganism is provided by forming a mixture of a biocatalyst solution and a silk fibroin solution, forming a membrane by casting the mixture, drying the membrane and mechanically treating the resultant membrane at a temperature and humidity to produce the membrane in .beta.-form and to structurally stabilize the membrane. Mechanically treating may be by stretching or compressing. A biocatalyst sensor is formed by coating an electrically conductive substrate with a gas-permeable layer and coating the gas-permeable layer with the silk fibroin membrane containing an entrapped biocatalyst.