Comprehensive plasma observations carried out on board the Hoes 2 satellite have provided the first systematic description of plasmas in the distant polar magnetosphere. These observations have revealed the presence of a persistent layer of tailward-flowing magnetosheathlike plasma inside of and adjacent to the magnetopause. This region has been termed the 'plasma mantle.' The mantle has been found to extend over the entire surface of the magnetosphere tailward of the polar cusp and northward of the plasma sheet. Vela observations of a 'magnetotail boundary layer' obtained in the vicinity of the plasma sheet by Hones and coworkers refer to the same phenomenon. The salient features of the plasma mantle as provided by Heos measurements from February to December 1972 can be summarized as follows: (1) The mantle was found to be present in over 70% of the passes through the polar magnetosphere in the region described above. (2) Its thickness varies greatly, ranging up to >4 RE, and does not appear to depend significantly on position or the state of the magnetosphere as measured by Kp. (3) A tailward-directed bulk flow parallel to the local terrestrial magnetic field was nearly always distinctly measurable. It was found to lie usually between 100 and 200 km s-1 ans was always less than the concurrent flow speed in the nearby magnetosheath. (4) The flow speed in the mantel is positively correlated with flow speeds in the magnetosheath and solar wind but depends only very weakly, if at all, on distance from the polar cusp, i.e., on XGSM. (5) A narrow region of low density and/or low flow speed plasma, i.e., a 'gap,' 0.1-0.2 RE thick, is frequently observed between the plasma mantle and the magnetopause. (6) The mantle protons are normally significantly cooler along B than perpendicular, i.e., T∥<T⊥. (7) The proton density, temperature, and bulk speed all tend to decrease gradually with depth inside the magnetopause, but this trend can at times be obscured by fluctuations and magnetopause motions. At the inner edge of the mantle the proton distribution is often very narrow in both energy and angle, i.e., relatively cold, before it finally disappears below the 100-eV threshold of the instrument. It is concluded, after an appraisal of several possible mechanisms, that the most probable cause for the formation of the mantle is the day side merging of terrestrial and interplanetary field lines.