In this work, we report on dislocation densities in MBE grown ZnSe/GaAs/GaAs heterostructures of different epilayer thickness. We examined the full-width at half-maximum of the reflection peaks obtained by high-resolution X-ray diffraction. We observed three regimes of dislocation generation. The first regime is present for samples of subcritical thickness, where only stacking faults are present and their formation depends on the conditions of preparation of the substrate and the initial conditions of growth. The second regime exists for samples with layer thickness greater than the critical thickness and thinner than a threshold thickness ht≅0.3 μm, where a large amount of misfit and threading dislocations are generated. The third regime starts for layer thickness greater than ht. In this regime the formation of dislocations is substantially slowed down and the dislocation density follows a 1/h dependence as predicted by the glide model. Comparing dislocation densities obtained by X-ray diffractometry with the pit density revealed by chemical etching of one sample and those observed by transmission electron microscopy of two samples, agreement exists among them within the same order of magnitude.