Using weak beam dark-field transmission electron microscopy, we have identified, by reference to stacking fault trapezoids, an additional crystal defect, a stacking fault tetrahedron, at the interface of ZnSe/GaAs(001) epilayers. Unlike the other stacking faults which are basically of uniform size, the size of stacking fault tetrahedra can vary by a few times, with a typical value of less than 10 nm. The stacking fault tetrahedron is a closed defect and does not extend very far from the interface. The density of the stacking fault tetrahedra is at least as high as that of the dominant stacking faults trapezoids. ©1997 American Institute of Physics.