The three-dimensional structure of the alkaline protease of Pseudomonas aeruginosa, a zinc metalloprotease, has been solved to a resolution of 1.64 A by multiple isomorphous replacement and non-crystallographic symmetry averaging between different crystal forms. The molecule is elongated with overall dimensions of 90 x 35 x 25 A; it has two distinct structural domains. The N-terminal domain is the proteolytic domain; it has an overall tertiary fold and active site zinc ligation similar to that of astacin, a metalloprotease isolated from a European freshwater crayfish. The C-terminal domain consists of a 21-strand beta sandwich. Within this domain is a novel 'parallel beta roll' structure in which successive beta strands are wound in a right-handed spiral, and in which Ca2+ ions are bound within the turns between strands by a repeated GGXGXD sequence motif, a motif that is found in a diverse group of proteins secreted by Gram-negative bacteria.