We used the spin-Hamiltonian method for the analysis of the electron paramagnetic resonance (EPR) spectrum of Fe3+ as a probe ion in (NH4)2AlF5·H2O single crystalline basic material. The theoretical expressions for the magnetic field (at which the fine structure transition lines appear) versus the angle between the magnetic field and the axis of symmetry of the magnetic complex are also given. These values were calculated by applying the perturbation theory to the second-order terms. From the experimental results (at 300 K and 9.21 GHz), the spin-Hamiltonian parameters were deduced:D=(668±10)·10−4 T,E=(−56±10)·10−4 T,a=(−54±10)·10−4 T,F=(30±10)·10−4 T. An isotropic superhyperfine structure was evidenced for the five fluorine ions. The obtained EPR data were used to determine the local symmetry of the Al3+ ion. A good agreement with X-ray diffraction measurements was found.