Dislocations in thin platelets of aluminum nitride grown from the vapor phase appear to be of two types. Some are dissociated into partials of the Shockley type; others are undissociated. A model is given for both types. The stacking fault associated with the dissociated dislocations consists of one lamella of the sphalerite structure. The stacking fault energy is deduced from the width of the ribbons and from the shape of the extended nodes; its value is &ggr; ≈ 4 ergs/cm2.