[en] A theoretical model of nucleation at the lateral surface of whisker nanocrystals during molecular-beam epitaxy and other high-vacuum deposition procedures is suggested. A model solution is obtained, which allows the calculation of the shape of whisker nanocrystals in relation to the diffusion flux of adatoms from the substrate surface to the base of the crystals. The temperature dependence of the nucleation effect is treated theoretically. It is shown that, at lowered surface temperatures, nucleation at the side walls results in the formation of cone-shaped crystals, with the length decreasing with decreasing temperature. The theoretical results are compared, at a qualitative level, with the experimental data for the GaAs whisker nanocrystals grown by molecular beam epitaxy on the Au-activated GaAs(111)As surface in the temperature range 400-600 deg. C