We interpret the low-frequency wave observations of the AMPTE/IRM mission in the downstream region of a marginally supercritical, quasi-perpendicular bow shock. A companion paper on ion heating comprehensively describes the corresponding plasma and wave magnetic field measurements. A simple model of the shocked plasma consisting of a background hydrogen magnetoplasma with anisotropic core protons permeated by a hotter dilute beam of anisotropic protons reproduces the basic features of the wave polarization and frequency spectra: wave activity below the proton cyclotron frequency, sometimes displaying a double-humped structure, with unusual (in space plasmas outside the magnetosphere) dominance of (plasma frame) left-hand-polarized modes. The core and beam anisotropies stimulate electromagnetic (left-hand) ion cyclotron waves in distinct spectral regions, whereas the large downstream Alfv¿n speed (with respect to the beam drift speed) precludes an efficient cyclotron resonant generation of the right-hand magnetosonic modes. ¿ Copyright 1990 by the American Geophysical Union