The ability to predict diffusion coefficients is important in the design, analysis, and operation of plasmid downstream processing operations such as membrane and fixed-bed chromatography. A correlation is proposed to predict the diffusion coefficient, D, of supercoiled plasmid DNA molecules in dilute solutions on the basis of the molecular weight, M, or size. Experimental data (18 points) collected from the literature confirmed the proposed variation of D with plasmid molecular weight as D proportional to M-2/3, for molecules within the 1,800-287,100 base-pair range. The correlation was able to estimate the available experimental results with an average error of 6.3%.