Alkaline protease was immobilized on vermiculite by covalent linkage. When compared to the free enzyme the immobilized form exhibited a lower specific activity towards high molecular-weight substrates but a broader pH tolerance and higher thermal stability. The pH optima of the immobilized alkaline protease was shifted towards the alkaline side by one unit. However, there was no change in optimum temperature between the free-and immobilized alkaline proteases. The immobilized alkaline protease exhibited good storage stability and re-usability.