The morphological evolution of crystals that grow from an incongruent vapor by the surface nucleation and screw dislocation mechanisms is simulated with a Monte Carlo model. The model combines volume transport, based on a modified diffusion-limited aggregation process, with anisotropic surface kinetics without a SOS restriction. It is found that for a given set of simulation parameters and symmetry of the lattice, there is a critical size beyond which a crystal cannot retain its stable, macroscopically faceted shape. This critical size scales linearly with the mean free path in the vapor. While surface diffusion is seen to stabilize the growth morphology on the scale of the surface diffusion length, volume diffusion is always destabilizing. Surface roughness increases with increase in growth temperature and supersaturation, which reduce the anisotropy in kinetics through thermal and kinetic roughening, respectively. For the screw dislocation mechanism, that can dominate at low temperature and supersaturation, we find that the combined effect of bulk and surface diffusion reduces the terrace width of a growth spiral in its center region. At elevated temperature and supersaturation normal growth can dominate in corner and edge regions of a crystal, while the spiral growth mode prevails in the center of a facet. © 1991.