The effect of nonhydrostatic stresses on the solid phase epitaxial growth rate of crystalline Si(100) into self-implanted amorphous surface layers has been measured. Uniaxial stresses of up to 6 kbar (0.6 GPa) were attained by bending wafers over SiO2 rods and annealing at a temperature too low for plastic deformation to relieve the stress in the crystal, but high enough for solid phase epitaxial growth to proceed. The growth rate on the tensile side was greater than that on the compressive side of the wafer, in marked contrast to the enhancement observed from hydrostatic pressure. The phenomenology of an “activation strain”, the nonhydrostatic analogue of the activation volume, has been developed to characterize the results. Combined with the measurement of the activation volume, the measurement reported here permits us to characterize to first order the entire activation strain tensor corresponding to the transition state for solid phase epitaxy of Si(lOO). We conclude that the transition state for this process is “short and fat”; that is, the fluctuation to the transition state involves an expansion in the two in-plane directions and a contraction in the direction normal to the surface large enough to make the overall volume change negative. The symmetry of the measured activation strain tensor is inconsistent with all bulk point defect mechanisms for solid phase epitaxy. The relevance of the activation strain formalism to heteroepitaxy and vapor phase epitaxy is discussed.