Guamerin, a small peptide inhibitor of the serine protease from Hirudo nipponia, was expressed in yeast and crystallized using the vapor diffusion method, with MPD as precipitant. The crystal was found to belong to the monoclinic P2(1) space group with unit cell parameters a=136.06, b=206.59, c=227.39 A, beta=105.03 degrees. The guamerin/bovine pancreatic chymotrypsin complex was also crystallized using PEG 8K as precipitant. The space group was identified as P2(1)2(1)2(1) with unit cell parameters of a=44.01, b=44.30, c=122.47 A. The diffraction data of the complex were collected up to a resolution of 2.4 A using a synchrotron-radiation source under cryogenic condition.