The ionic strength dependences of the translational diffusion coefficient and the first internal mode relaxation time of monodisperse 2311 base pair relaxed circular DNA were studied by dynamic light scattering. Intensity autocorrelation functions were analyzed using the inverse Laplace transform method (CONTIN). Above the lowest scattering angle studied (33 degrees), generally two peaks were found in the distribution of decay rates. The translational diffusion coefficient, corresponding to the slower of the peaks, at a low DNA concentration of 100 mu g mL(-1) was found to be (4.52 +/- 0.2) X 10(-8) cm(2) s(-1), nearly independent of added NaCl from 1 to 0.001 M using a buffer of 10 mM Tris.HCl, 1 mM EDTA, pH 8. The mean relaxation frequency of the first internal mode, extracted from the faster of the two peaks, decreases by a factor of 2 as the added NaCl is decreased to 0 from 1 M. The amplitude of the faster mode, when plotted as a function of the square of the reduced scattering vector length, q(2)R(g)(2), lies on the same curve at all ionic strengths studied. Calculation of the radius of gyration, R(g), as a function of ionic strength based on a scaling argument shows about a 25% increase as the added salt is decreased from 1 M. An "extraordinary mode" is observed when the Tris content is reduced to 1 mM with both slow and fast diffusive modes observed.