Dynamic light scattering-photon correlation spectroscopy (PCS) has been used to study the dynamics of a 2311 base pair (bp) superhelical plasmid DNA in dilute and semidilute solutions. In dilute solution, translational and rotational/internal modes could be resolved. Assuming that the DNA is a rigid rod, the translational diffusion coefficient determined at infinite dilution gives cylinder dimensions consistent with a calculated length L of 230 nm and a diameter of 13.4 nm. If the rotational/internal mode is interpreted as rotation of a rigid rod, the derived rotational diffusion coefficient is consistent with a shorter rod of a smaller length to diameter ratio. The dilute and semidilute regimes are clearly distinguished by a break in the concentration dependence of the mutual diffusion coefficient. The transition concentration, using the L given above, was found to lie at cL(3) between 3 and 5. In the semidilute regime, a slow mode was detected in addition to the main diffusion mode. The relative amplitude of the slow mode increases with increasing concentration until it is the dominant mode in the DLS frequency distribution. The slow mode frequency decreases with increasing concentration. The characteristics of the slow mode are not in accord with those of the slow mode predicted by the spinodal decomposition theory of Doi, Shimada, and Okano (DSO) for stiff rodlike polymers. The observed mode is consistent with the formation of aggregates in the semidilute regime. The mutual diffusion coefficient in the semidilute regime shows a linear concentration dependence, which is in qualitative agreement with the predictions of the DSO theory. The slope of this linear concentration dependence, however, differs markedly from the theoretical prediction assuming a concentration-independent self-diffusion coefficient. In addition, no deviation from quadratic scattering vector length dependence of the mutual diffusion mode, as predicted by the DSO theory, is observed. The rotational diffusion/internal relaxation frequencies appear to decrease with increasing concentration. However, the rather large scatter in the data (due to the relatively small contribution of this mode to the spectrum in the presence of the slow mode) prevented a quantitative comparison with existing theories.