The escape of hydrogen and deuterium from the Earth is regulated by eddy transport of hydrogenic gases (e.g., water vapor, methane, and molecular hydrogen) from the troposphere, through the stratosphere, and into the mesosphere and lower thermosphere where photochemistry produces atomic forms of hydrogen and deuterium. Farther up, escape creates a deficit of these atoms relative to diffusive equilibrium, and this in turn leads to upward fluxes of both species to balance escape. A Monte Carlo exosphere simulator program, which has evolved through numerous applications to the moon and terrestrial planets, has been adapted to a coordinated study of the dynamics of D and H in the terrestrial exosphere, plasmasphere, and polar wind. Results show that while the escape of hydrogen is compatible with Hunten's limiting flux at the homopause (about 2.5¿108 cm-2 s-1), the upward flow of deuterium is suppressed by slow escape, which in turn causes an exospheric buildup. The global average escape flux of deuterium is 55% (¿10%) of its limiting flux, or 1.5¿104 cm-2 s-1. From an isotopic perspective, the escape rate of D relative to H corresponds to about 39% of the D/H ratio in SMOW. ¿ 1998 American Geophysical Union