X-ray double-crystal rocking curve and topography measurements have been used to obtain a detailed evaluation of the crystalline quality of ZnSe(001) films grown by molecular-beam epitaxy on GaAs(001). Rocking curves were measured for three incident x-ray wavelengths to identify and determine the contribution of specific physical defects to the linewidth through their wavelength dependence. X-ray topography was used to obtain a very direct, nondestructive depth-resolved record of the physical structure of both epilayer and substrate through the use of various asymmetric reflections having different penetration depths for dynamical scattering. We find that the GaAs substrate is strained by the misfit to the ZnSe epilayer. The epilayers exhibit a fine microstructure, consisting of slightly misoriented mosaic blocks, which is very uniform both laterally and throughout the thickness of the film. The rocking curve linewidth for the epilayer is dominated by the lattice tilt between these blocks. The resultant dislocation density as estimated from the linewidth is on the order of 1.4×108 cm−2.