We calculate the proton-deuterium (p-D) fusion reaction rate at low energies (E ≤ 2 keV in the center-of-mass frame) for a Maxwell-Boltzmann velocity distribution and compare it to those for other reactions involving hydrogen isotopes. It is shown that p-D fusion dominates competing reactions for E ≤ 8 eV in the center-of-mass frame. The implications for various physical processes are discussed.