The present Letter reports on self-diffusion in amorphous silicon. Experiments were done on Si-29/Si-nat heterostructures using neutron reflectometry and secondary ion mass spectrometry. The diffusivities follow the Arrhenius law in the temperature range between 550 and 700 degrees C with an activation energy of (4.4 +/- 0.3) eV. In comparison with single crystalline silicon the diffusivities are tremendously higher by 5 orders of magnitude at about 700 degrees C, which can be interpreted as the consequence of a high diffusion entropy.