We studied the propagation of a microwave signal in Fe/Ag-GaAs waveguide structures. In an ultrahigh frequency ferromagnetic resonance measurement, maximum coupling and thus strong attenuation of the microwave power occur near the ferromagnetic resonance frequency f(res) of Fe film. Magnetostatic wave (MSW)-based microwave devices have been achieved in the form of a bandstop filter utilizing single crystal Fe/Ag structures grown on GaAs (100) substrate. MSW-based microwave devices possess the unique capability of electronically tunable operating carrier frequency via a magnetic field. The peak absorption frequency for an Fe easy axis case has been tuned in a range from 10.6 to 36 GHz. For a hard axis case, the tuning frequency ranges from 8 to 22 GHz. The experimental results show that Fe/Ag-GaAs waveguide structures are very promising systems for use in future microwave magnetoelectronics as they have well-defined magnetic properties, as well as favorable electrical properties.