A study has been made of the characteristics of MHD waves and associated density fluctuations in the Earth's foreshock. Data from two periods were analyzed. In one period the waves were circularly polarized, of 20% fractional amplitude, and propagating in a plasma with &bgr; less than one. In the second period the waves were elliptically polarized, of 50% fractional amplitude, and the plasma &bgr; was in excess of unity. The following results emerge. (1) For both periods there is a prominent feature in the density power spectrum at the same frequency as that characterizing the dominant magnetic field fluctuations. We attribute this feature to oblique wave propagation. (2) Ponderomotive effects are responsible on both days for the generation of density fluctuations on scales comparable to the wave packet sizes. There is observational evidence for modulational instability of the MHD waves on the day with &bgr; in excess of unity. (3) Evidence for a parametric decay instability was not observed, but conditions were only marginally favorable for such an observation. (4) Generation of density fluctuations by linearly polarized waves does not seem to be an important contributor to the observed density fluctuations. ¿ American Geophysical Union 1988