Aqualysin I, is a subtilisin-like serine proteinase, from the thermophilic bacterium Thermus aquaticus. It is predicted that the enzyme contains a salt bridge, D17-R259, connecting the N- and C-terminal regions of the enzyme. Previously we reported on the stabilizing effect of the incorporation of a salt bridge at a corresponding site in VPR, a related cold adapted enzyme from a marine Vibrio sp. Here we describe the effect of the reverse change, ie the elimination of the salt bridge on the thermal stability and kinetic properties of aqualysin I. Deletion of the putative salt bridge in the D17N mutant of the enzyme destabilized the enzyme at 8-9 ° C in terms of T50%, determined by thermal inactivation and over 4 ° C in Tm, as measured from melting curves of the inhibited enzyme. The mutation, however, had no significant effect on the kinetic parameters of the enzyme under standard assay conditions.