Observations obtained upstream of the earth's bowshock with the LASL/MPI plasma instruments and the UCLA magnetometers on ISEE-1 and 2 have revealed a striking relationship between the presence of low-frequency fluctuations in solar wind density and field strength and the different types of distribution functions of upstream ions. Waves are absent when the ions have the beamlike distribution of the ''reflected'' ions. Large-amplitude waves are present only in conjuction with the ''diffuse'' ions, which are characterized by flat energy spectra and broad angular distributions. The waves are largely compressive, showing very good correlation between oscillations in magnetic field strength and plasma density.