PorM is a membrane protein involved in the assembly of the type IX secretion system (T9SS) from Porphyromonas gingivalis , a major bacterial pathogen responsible for periodontal disease in humans. The periplasmic domain of PorM was overexpressed in Escherichia coli and purified. A fragment of the purified protein was obtained by limited proteolysis. Crystals of this fragment belonged to the tetragonal space group P 4 3 2 1 2. Native and MAD data sets were recorded to 2.85 and 3.1 Å resolution, respectively, using synchrotron radiation.