The electrophoretic migration of 40 and 90 kbp supercoiled (sc) double-stranded (ds) plasmid DNA in dilute and entangled solutions of linear hydroxyethyl cellulose (HEC, M-n, = 438 800) was visualized by video fluorescence microscopy at 60-222 frames/s. The sc-DNA molecules migrated as elastic rods with stretch-contraction cycles directed along the electric field axis. The elastic rod electrophoretic behavior of sc-DNA is completely different from the familiar ran;random coil electrophoretic dynamics of linear ds-DNA. Extension-contraction cycles were more periodic for sc-DNA than for linear ds-DNA. The consequences of the different sc-DNA and linear ds-DNA electrophoretic dynamics on mobility dispersion are discussed.