76As isotope has been diffused along stair rod dislocations associated with stacking faults in Si epitaxial layers. Diffusions were performed in a two-temperature zone furnace using neutron-activated elemental As as the diffusion source. Uniform densities of dislocations (>106/cm2), with axes intercepting the diffusion surface, were obtained following the growth of epitaxial layers on Si substrates whose surfaces were damaged by ion implantation to high dosages. Sections were taken by an anodizing and stripping technique and their activity was determined by liquid scintillation methods. The resulting profiles revealed a composite shape indicative of normal lattice diffusion near the surface together with a deeply penetrating portion which can be unambiguously associated with rapid diffusion along dislocations. For temperatures in the range 950°-1050°C, the intrinsic diffusivity of As in the lattice is given by Di=(0.51+0.19 -0.14cm2/sec) exp (3.53±0.03 eV/kT) and for the dislocations KδDd=((9.4±1.6)x10-8cm3/sec) exp (2.56±0.02 eV/kT).