The Verdet constants of samples of water containing 0, 31.1, and 99.7 percents of heavy hydrogen atoms have been measured for the wave-lengths $\ensuremath{\lambda}5893$ and $\ensuremath{\lambda}5460.7\mathrm{A}$ at 20\ifmmode^\circ\else\textdegree\fi{}C. The results are given in tabular form. From these data the Verdet constant of pure heavy water (${\mathrm{H}}^{2}$$_{2}\mathrm{O}$) is found to be 3.93 percent less than that of ordinary water for both wave-lengths. Non-linearity of the Verdet constant-concentration relation leads to the calculation of an apparent Verdet constant for ${\mathrm{H}}^{1}$${\mathrm{H}}^{2}$O which is 0.85 percent less than the Verdet constant of ordinary water. Values of the relative molecular rotatory power and of the magnetic rotatory dispersions are also given.