Transitions between isotropic and liquid crystalline phases of concentrated solutions of DNA with an average contour length (500 A) near the persistence length were examined in 0.01 M supporting 1:1 electrolyte (predominantly NaCl). A quantitative phase diagram describing the transitions occurring over a DNA concentration range from 100 to 290 mg/mL and temperatures from 20 to 60 degrees C was constructed from solid-state 31P-nmr data and examination of the morphologies of the mesophases by polarized light microscopy. Three anisotropic phases were observed in solutions with DNA concentrations of 160-290 mg/mL: an unidentified, weakly birefringent phase termed "precholesteric," a true cholesteric phase with pitch approximately 2 microns, and a third, presumably more highly ordered phase. Comparison with previous studies showed that the critical concentration for anisotropic phase formation and the nature of the phases formed by these DNA molecules are not strongly affected by decreasing the supporting electrolyte concentration from approximately 0.2 M to 10 mM. There are, however, profound effects of decreasing the supporting electrolyte concentration on the width of the transition from isotropic to totally anisotropic solutions, and the nature of the transitions between phases. Decreasing the supporting electrolyte concentration significantly increases the concentration range of persistence of the isotrophic phase, and results in the formation of triphasic solutions (isotropic and two liquid crystalline phases). Values of the critical DNA concentrations for anisotropic phase formation from the theory of A. Stroobants et al. [(1986) Macromolecules 19, 2232 to 2238] were found to be significantly lower than the observed values for any reasonable estimate of the effective radius, probably because of the relatively short lengths of DNA fragments examined in the present study. Comparison of the experimentally determined DNA concentrations required for anisotropic phase formation with the values predicted from Flory's lattice statistics theory, which explicitly considers the rod length, permitted estimation of the effective DNA radius. The estimated radius was inconsistent with effective radii calculated from Poisson-Boltzmann (P-B) theory based on a supporting electrolyte concentration of 10 mM, but was in fair agreement with P-B theory assuming that Na+ DNA contributes approximately 0.24 Na+ counterions/nucleotide to the effective free sodium ion concentration.