The sequence complexity of several plasmids of Bacillus megaterium has been determined by reassociation kinetics, both optically and by the S1 nuclease method. G + C values calculated from T m's obtained at several salt concentrations were compared to G + C values calculated from buoyant densities determined by isopycnic centrifugation. The G + C values for each plasmid are not substantially different from each other or from the chromosome, and the comparison of values calculated by the two methods shows that there are few, if any, modified or unusual bases present. Reassociation kinetics imply that each plasmid size class is unique in nucleotide sequence and that they contain no detectable high-complexity chromosomal DNA. Two plasmid species, the 4.0- and 6.2-md species, contain internal AT-rich regions. The role of dioxane in revealing such intramolecular heterogeneity is discussed.