The sedimentation rate of large, linear DNA molecules has been shown to be rotor speed dependent (Rubenstein and Leighton, Biophys. Chem. 1 (1974)). In this communication we report the first studies designed to measure the rotor speed effect with a homogenous, linear viral DNA larger than bacteriophage T2 DNA. We also report the first studies using a homogenous, circular episomal DNA of known molecular weight. For this circular DNA a small rotor speed effect, previously unsuspected, was discovered.