The properties of the fcc to hcp transformation in cobalt single-crystal whiskers have been determined. Studies of the crystallographic properties and dislocation structure using optical, standard x-ray diffraction, and high-resolution x-ray diffraction topographic techniques are herein reported. It is shown that the atomic movements associated with the transformation in cobalt initiate at the most highly strained regions in the crystal. The strain configuration and the crystal boundaries act as a combined mechanism restricting the transformation to a particular set of {111} planes. When the strain distribution is nonuniform throughout the crystal, localized regions within the crystal utilize different sets of {111} transformation planes. The transformation from the fcc to hcp phase is accomplished by the glide of Shockley partial dislocations through the lattice. Frank partial dislocations remain pinned in the lattice. These partials are formed from the extension of total dislocations. Martensitic platelets result from either the glide of Shockley partial dislocations which have different slip vectors in parallel and adjacent sections of the crystal or the pinning of Frank partial dislocations which form the boundary of faulted regions. The platelets intersecting the crystal surfaces produce observable slip bands.