The crystal structure of Serratia protease from Serratia sp. E-15 was solved by the single isomorphous replacement method supplemented with anomalous scattering effects from both the Zn atom in the native crystal and the Sm atom in the derivative crystal, and refined at 2.0AÅ resolution to a crystallographic R-factor of 0.194. The enzyme consists of N-terminal catalytic and C-terminal β-sandwich domains, as observed in alkaline protease from Pseudomonas aeruginosa IFO3080. The catalytic domain with a five-stranded antiparallel β-sheet and five α-helices shares a basically common folding topology with those of other zinc metalloendoproteases. The catalytic zinc ion at the bottom of the active site cleft is ligated by Hisl76, Hisl80, His186, Tyr216, and a water molecule in a distorted trigonal-bipyramidal manner. The C-terminal domain is a β-strand-rich domain containing eighteen β-strands and a short α-helix, and has seven Ca2+ions bound to calcium binding loops. An unusual β-sheet coil motif is observed in this domain, where the β-strands and calcium binding loops are alternately incorporated into an elliptical right-handed spiral so as to form a two-layer untwisted β-sandwich structure. The Ca2+ ions in the C-terminal domain seem to be very important for the folding and stability of the β-sheet coil structure.