Examples are cited for the detection of dislocations, subgrain boundaries and inhomogeneities of dopant in GaP moaocrystals by means of the cathodoluminescence (CL) modo of the scanning electron microscope (SEM). CL micrographs contain valuable information about the density of recombination centres with the rcstriction that the kind of a crystal imperfection in generality is not given by this method alone. The method of CL in the SEM is able to resolve e.g. dislocation densities greater than 104 cm -2. In the case of inhomogeneities of dopant CL generally rcpresents the only possibility of sensitive detection with high lateral resolution.