We present millimeter-wave observations of several molecular ions in the disk around the pre-main-sequence star DM Tau and use these to investigate the ionization fraction in different regions of the disk. New Submillimeter Array (SMA) observations of H2D J = 11, 0-11, 1, N2H J = 4-3, and CO J = 3-2 are presented. H2D and N2H are not detected and using the CO 3-2 disk size the observations result in an upper limit of are not detected and using the CO 3-2 disk size the observations result in an upper limit of -1 for both lines, a factor of 2.5 below previous single-dish H2D observations. Assuming LTE, a disk midplane temperature of 10-20 K and estimates of the H2D o/p ratio, the observed limit corresponds to N _H_2D^ o/p ratio, the observed limit corresponds to N _H_2D^ -2. We adopt a parametric model for the disk structure from the literature and use new IRAM 30 m telescope observations of the H13CO J = 3-2 line and previously published SMA observations of the N2H J = 3-2, HCO J = 3-2, and DCO J = 3-2 lines to constrain the ionization fraction, xi , in three temperature regions in the disk where theoretical considerations suggest different ions should dominate: (1) a warm, upper layer with T >20 K where CO is in the gas phase and HCO is most abundant, where we estimate xi ~= 4
