Bacterial isolates from the gland of Deshayes of the marine shipworm (Psiloteredo healdi) produced extracellular protease activity when cultured with 1% cellulose. A protease with a relative molecular mass of 36,000 daltons as determined by SDS-PAGE and a pl of 8.6 was isolated from the medium and purified to electrophoretic homogeneity. No carbohydrate appeared to be associated with the protein. The enzyme was activated and stabilized by relatively high salt concentrations (> 0.2 M). Below 0.1 M salt, significant protein aggregation occurred, as well as autohydrolysis of the protease, both of which resulted in the loss of activity. The specific activity of the enzyme was 65,840 proteolytic units/mg with azocasein substrate of optimal temperature (42-degrees-C), pH (9.0), and salt concentration (0.20 M NaCl). The activity was stable up to 40-degrees-C, from pH 3.0 to pH 11.9, and from 0.1 M to 3.5 M NaCl. These stabilities, as well as the protease's stability in the presence of chelators, oxidizing agents, and heavy metals, suggest the enzyme has potential for use in relatively low temperature (40-degrees-C) industrial applications.