Achromobacter protease I (API), a serine protease, shows an order of magnitude higher activity than bovine trypsin. The optimum pH of mutant enzymes with His210 replaced by Ser (H210S) and Trp169 replaced by Phe (W169F) has been shown to shift from approximately pH 9 (wild-type enzyme) to approximately pH 7 while retaining high activity. The mutants were crystallized by the hanging-drop vapour-diffusion technique with 2 M ammonium sulfate as the precipitant. The space group of the W169F mutant crystal was P1, with unit-cell parameters a = 42.6, b = 34.7, c = 69.5 Å, α = 91.8, β = 97.5, γ = 89.9°, while the space group of the H210S mutant crystal was P2(1), with unit-cell parameters a = 42.4, b = 34.2, c = 67.7 Å, β = 97.6°. Diffraction data were collected from W169F and H210S crystals to resolutions of 2.0 and 2.3 Å, respectively.