The thermostability of an alkaline protease, AprP from Pseudomonas sp. KFCC 10818, was improved by replacing Ser307 and Ser331 at the autoproteolytic cleavage sites with various amino acids. Six mutant enzymes were purified and characterized. Two of these had half-lives four and three times longer than the wild-type protease at 55degreesC in the presence of 1 mM CaCl2. Three mutant enzymes had half-lives twice as long as the wild-type under the same condition.