it is generally accepted that linear polymers are larger, such as with regard to the end-to-end distance and gyration radius, than the corresponding circular polymers. We measured the Brownian motion of individual linear and circular 106-kbp DNA molecules using fluorescence microscopy. Unexpectedly, the hydrodynamic radius of linear DNA was markedly smaller than that of circular DNA. (c) 2005 Elsevier B.V. All rights reserved.