High-resolution X-ray diffraction is commonly used to measure the misfit strain and to determine the unstrained lattice parameter of epitaxic semiconductor layers assuming tetragonal distortion. In this paper a method is developed which links the measured peak separations in X-ray rocking curves to the strain tensor without an assumption of a layer symmetry. For the system (Zn,Mn)Se on thick ZnSe buffer layers deposited by MBE (molecular beam epitaxy) on GaAs substrate, the six components of the reciprocal metric tensor of each layer were determined by a least-squares algorithm from a set of 12 different reciprocal-lattice vectors. A principal-axis transformation of the calculated strain tensor of the (Zn,Mn)Se layer shows that the deformation in the interface plane nearly parallel to the [110] and [110] directions differs by more than 15%. The different critical thicknesses for the relaxation in these two directions are determined to be 36 and 87 nm, respectively. The deviation from tetragonal symmetry is a function of the epilayer thickness and reaches a maximum at about 200 nm.