The two-domain, 325 residue outer membrane protein OmpA is one of the most abundant proteins of Escherichia coli, playing a role in the maintenance of the integrity of the cell surface. The N-terminal domain, consisting of about 170 amino acid residues, is embedded in the membrane, presumably in the form of a beta-barrel consisting of eight amphipathic transmembrane strands. Pairs of these proposed transmembrane strands were permuted at the DNA level, in order to dissect the process of membrane assembly. All three possible circular permutations led to variants, which were, in comparison with the wild-type protein, less efficiently assembled. In contrast, no membrane assembly could be detected in any of 18 non-circularly permuted variants. We take this as an indication that the "right" (wild-type) order of beta-strands is a necessary and sufficient prerequisite for at least partially successful membrane assembly. This may be the consequence of packing constraints and/or a failure to adopt the wild-type arrangement of beta-strands, which require crossing of the periplasmic turns.