We have determined the stability of the dense hydrous magnesium silicate phase D in a Mg2SiO4+20.5 wt% H2O composition between 16 and 25 GPa at 900¿--1400 ¿C. Phase D coexists with superhydrous phase B and a Mg-rich liquid to temperatures of 1000 ¿C at 17 GPa and 1400 ¿C at 26 GPa. Experiments in an externally heated diamond-anvil cell confirm that phase D is stable to pressures of at least 50 GPa at 930 ¿C. From static compression measurements, the zero-pressure bulk modulus of phase D was determined as 200¿7 GPa. We also present evidence that phase D is identical to phase F <Kanzaki, 1991> and phase G <Kudoh et al., 1997a; Ohtani et al., 1997>. The high-pressure and high-temperature stability of phase D makes it eminently suitable as a hydrous phase which is stable within subducting lithosphere, in the transition zone, and in the lower mantle. The positive pressure-temperature stability slope of phase D, determined in this study, indicates that it is potentially stable at temperatures much greater than 1400 ¿C in the lower mantle. ¿ 1998 American Geophysical Union