The adhesin involved in diffuse adherence (AIDA-I) from Escherichia coli belongs to the group of autotransporters, specifically the type Va secretion system (T5aSS). All autotransporter systems contain a C-terminal β-domain, which forms a barrel-like structure in the outer membrane with a hydrophilic pore allowing passenger translocation across the outer membrane. The passenger domain harbours the biological activity in the extracellular space and functions, for example, as an adhesin, an enzyme and a toxin. The exact transport mechanism of passenger translocation across the outer membrane is not clear at present. Thus, structure determination of the transport unit of AIDA-I could provide new insights into the transport mechanism. Here, the purification, crystallization and preliminary X-ray crystallographic studies of the transport unit of AIDA-I are reported.