Using microdiffraction techniques at the Advanced Photon Source, we have found evidence for local molecular alignment within the mesoscopic fibers of DNA–cationic membrane complexes, a system originally conceived as gene delivery vectors. Furthermore, these mesoscopic structures can be manipulated by using lithographically patterned microchannel arrays, so that the optical axes of individual fibers are aligned parallel to the channels. These observations suggest that DNA–membrane complexes may have additional technological applications, such as active electrophoretic media and templates for the fabrication of semiconductor superlattices and nanoporous materials with tunable pore sizes. ©1998 American Institute of Physics.