This paper considers the linear theory of electromagnetic ion beam, and ion ring-beam instabilities in a homogeneous Vlasov plasma. Propagation parallel or antiparallel to a uniform magnetic field and frequencies at or below the proton cyclotron frequency are considered. For parameters representative of the distant cometary environment, we show that instabilities with right-hand polarization in the zero momentum frame have larger growth rates than left-hand polarized instabilities at &agr; values up to 90¿ where &agr; is the angle between the solar wind velocity and the uniform interplanetary magnetic field. If both a proton beam and an oxygen beam are present with &agr;=0¿, two right-hand resonant instabilities may grow; these two modes are distinct and relatively independent of one another for a very wide range of proton/oxygen beam density ratios. ¿ American Geophysical Union 1988