Superhelical PM2 DNA can be photochemically modified by u.v. irradiation. The variation of S20,w with dose shows the following characteristics. There is a linear increase from 28 to 31s produced by a low dose of u.v. irradiation (4,000 ergs/mm2). A plateau in S20,w occurs between 4,000 and 10,000 ergs/mm2. The S20,w then increases when irradiation is increased to 56,000 ergs/mm2. Thymine dimers are introduced proportional to dose throughtout the range of exposure to u.v. light. Sedimentation velocity-dye titrations reveal anomolous behavior, i.e. apparent increases in superhelix density (sigma). However, the dye-buoyant density procedure showed no change in sigma under the same conditions. The most satisfactory model for the data is preferential photochemical modification of premelted (possibly hairpin) sites as a greater rate than the introduction of photoproducts into duplex sites. The origin of the anomoly in the sedimentation velocity dye titrations is still unclear.