The nucleation of perfect or partial dislocations from a surface-step of a f.c.c. or diamond-like material is frequently observed. In a recent paper, it is shown how the "stress—stacking-fault energy" plane can be divided into three zones where a partial or a complete dislocation will or will not nucleate. In compound semiconductors, dislocations can dissociate into α or β partial dislocations with mobilities appreciably different. In this paper, this effect is taken into account and yields to large modifications in the partial or perfect dislocations nucleation conditions. The case of GaAs is specially examined.