Omp32, the strongly anion-selective porin from Comamonas acidovorans, has been crystallized. Two crystal forms were observed, both of which belong to space group R3, but exhibit different cell dimensions a = b = 106.7, c = 140.6 A (crystal form I) and a = b = 87.1, c = 135.3 A (crystal form II) with one trimer per asymmetric unit. The crystals diffract to 2.2 and 2.3 A resolution, respectively. Omp32 was chemically modified by introducing negative charges through succinylation. The number and positions of the individual modifications were determined using mass spectrometry and X-ray crystallography. Chemically modified porins yielded crystals of a third form, also of space group R3 but with cell constants of a = b = 109.3 and c = 263.2 A (crystal form III), showing a virtually doubled c axis. Crystals of form III diffract to 3.5 A resolution.