In interpenetrating ion streams the left-handed circularly polarized electromagnetic waves propagating along the magnetic field can become unstable when either the relative speed between the streams or the ion temperature anisotropy exceeds a certain minimum value. The instability is excited only in a bounded range of wave numbers and is enhanced by an increase in ion number density and a decrease in electron temperature anisotropy. The solar wind double ion stream configurations observed at 1922 UT on May 20, 1971, are found to be unstable against this instability for ion densities of ?3 cm-3. The scale length of the instability is found to be of the order of the ion Larmor radius. The typical growth rates, which are ~0.05--0.1&OHgr;i, can distrupt the observed configurations in about 30 s.