&agr;‐Iron whiskers were grown under observationin situin an electron microscope by the thermal decomposition of iron carbonyl in a stream of hydrogen at 710°–820°C. As many as three growth periods with distinct growth rates were observed. An initial slow growth was postulated to be due to growth at the base, caused by localized stresses in the three‐dimensional iron deposit on the screen on which the whiskers were growing. Subsequent growth at the tip appeared to be limited by both the impingement rate of iron carbonyl and the surface flux of iron on the shaft of the whisker. The diffusion coefficient, calculated to account for the observed maximum growth rate, is in good agreement with the value given in the literature. Subsequent slower growth was accounted for by the incorporation of the iron contained in the Fe (CO)5which impinged on the tip alone. The morphological features observed and interpreted were tilting, bridge growth, lateral growth, curving, kinking, junction formation, and neck formation. It was shown that a whisker may grow by plastic deformation under tensile stress and simultaneous deposition on its shaft from a supersaturated gaseous atmosphere.