A new concept based on microstructuring techniques is presented to achieve size-dependent electrophoretic migration of DNA in free solution. Topographically structured microfluidic channels with periodical cavities in the range of the radius of gyration of the tested DNA molecules (< 3 mum) are produced by moulding of polydimethylsiloxane on a master wafer fabricated by contact lithography of SU-8 photoresist. The electrophoretic migration of single DNA molecules stained with the intercalator YOYO was investigated by real-time fluorescence video microscopy. It could be demonstrated for large DNA molecules that lambda-DNA (48 kbp) always migrated significantly faster than T2-DNA (164 kbp) in the two tested microchannel layouts. Integration into a microdevice for DNA separation in free solution is now in progress.