The porin from Paracoccus denitrificans ATCC 13543 was purified and crystallized. Two crystal forms were obtained from porin solutions with beta-d-octylglucopyranoside as detergent. Crystals of form I belong to the monoclinic spacegroup C2 with unit cell dimensions a = 112.2 A, b = 193.8 A, c = 100.5 A and beta = 129.2 degrees. There is 1 trimer per asymmetric unit. Crystals of form II are triclinic with a = 89.7 A, b = 98.8 A, c = 112.5 A, alpha = 112.5 degrees, beta = 101.8 degrees, gamma = 106.7 degrees (2 trimers per asymmetric unit). Both crystal forms diffract to 3 A.