The primary structure of the integral membrane protein porin from the purple bacterium Rhodobacter capsulatus was determined. The protein was cleaved with trypsin, CNBr and Asp-N protease. The peptides were isolated, sequenced and aligned to a total length of 301 residues with an Mr of 31536. The low isoelectric point of 3.9 is confirmed by the high excess of 34 Asp and 17 Glu (16.9%) over 10 Lys, 7 Arg and 2 His (6.3%). Overall sequence similarity to other porins is not evident when using sequence alignment programs. However, a partial relationship to Neisseria porins seems to exist. The established sequence has been used as the basis for a three-dimensional structure determination by X-ray diffraction at 0.18-nm resolution. The arrangement of the sequence in the 16-stranded β-barrel of porin is given. Some sequence-structure correlations are discussed.