The pair-breaking critical current density j_{d} of magnesium diboride was determined over its entire temperature range by a pulsed dc transport measurement. At fixed low values of current density j, the resistive transition temperature Tc shifts in the classic ΔT_{C}(j)/T_{C}(0) \propto -[j/j_{d}(0)]^{2/3} manner, with a projected j_{d}(0) \approx 2 \times 10^{7}$ A/cm^{2}. The directly measured j_{d}(0), from current-voltage ($I$-$V$)curves at different fixed temperatures, has a similar value and the overall temperature dependence j_{d}(T) and magnitude are consistent with Ginzburg-Landau theory. Keywords: pair, breaking, depairing, superconductor, superconductivity, flux, fluxon, vortex, mgb2

Submitted to Phys. Rev. Lett. Related preprints/reprints by our group can be found on http://www.physics.sc.edu/kunchur