A theoretical study of the axial propagation of plane-thermal-neutron waves in a heterogeneous system is performed in the framework of the P-1 approximation to the Boltzmann equation. The method is based on a modified form of heterogeneous reactor theory due to Feinberg and Galanin. The analysis predicts that the phase interference between the modes of propagation in the axial direction may give rise to resonances in the frequency response of the asymptotic moderator flux. A standing wave pattern is also predicted in the amplitude distribution of the oscillating part of the moderator flux in the axial direction. The relationships between the resonances and the system parameters are investigated. An experimental method that can be useful for the determination of the effective values of the diffusion parameters and the slowing down time is suggested. Numerical calculations for a heavy-water-moderated natural uranium system containing four identical fuel rods are presented in the frequency range from 0 to 1500 Hz. Two resonances are predicted in the transfer function of such a system in this frequency range. A comparison is made with the experimental results published in the literature for a similar system. The complex relaxation length for this system is also calculated numerically in order to study the effect of the resonances in the transfer function on the complex relaxation length. The results show existence of “loops” in the plot of the complex relaxation length.