AbstractCD spectra of calf thymus, C. perfringens, E. coli, and M. luteus DNA have been measured in the vacuum‐uv region to about 168 nm for the A‐, B‐, and C‐forms. The positive band at about 187 nm and the negative band at about 170 nm found for each type and form of DNA are sensitive to the source of the DNA and the base–base interactions of the double‐stranded helix. The A‐form spectra confirm that these bands are indeed sensitive to secondary structure. In the near‐uv, the CD of B‐form DNA is well analyzed as a linear combination of 27% A‐form and 78% C‐form. However, an analysis of the extended spectrum demonstrates that the near‐uv analysis is not correct. The extended analysis shows that the base–base interactions are similar for B‐ and C‐forms in solution, which implies that these two forms have nearly the same number of base pairs per turn. Various types of CD difference spectra are also discussed.