Carnein is an 80 kDa subtilisin-like serine protease from the latex of the plant Ipomoea carnea which displays an exceptional resistance to chemical and thermal denaturation. In order to obtain the first crystal structure of a plant subtilisin and to gain insight into the structural determinants underlying its remarkable stability, carnein was isolated from I. carnea latex, purified and crystallized by the hanging-drop vapour-diffusion method. A data set was collected to 2.0 Å resolution in-house from a single crystal at 110 K. The crystals belonged to the trigonal space group P3121 or P3221, with unit-cell parameters a = b = 126.9, c = 84.6 Å, α = β = 90, γ = 120°. Assuming the presence of one molecule per asymmetric unit, the Matthews coefficient is 2.46 Å3 Da-1, corresponding to a solvent content of 50%. Structure determination of the enzyme is in progress.