Unidirectional solidification of an aqueous NH4Cl solution has been simulated using a fully three-dimensional model for the strongly coupled transport of momentum, energy, and species in the mushy and melt regions of the solution. The predictions reveal the nucleation and development of pencil-like vertical channels in the mushy region and the complex, three-dimensional flow and thermosolutal conditions associated with channel formation. The number of channels is at a maximum shortly after nucleation and thereafter decreases with increasing time, as solidification progresses. Favorable agreement is obtained between three- and two-dimensional predictions of global parameters such as heat rate and mass fraction of solid formation.