Plasma electron observations made on board Mariner 10 during its three encounters with the planet Mercury show that the planet interacts with the solar wind to form a bow shock and a permanent magnetosphere. The observations provide a determination of the dimensions and properties of the magnetosphere, independently of and in general agreement with magnetometer observations. The magnetosphere of Mercury appears to be similar in shape to that of the earth but much smaller in relation to the size of the planet. The average distance from the center of Mercury to the subsolar point of the magnetopause is ~1.4 planetary radii. Electron populations similar to those found in the earth's magneto-tail, within the plasma sheet and adjacent regions, were observed at Mercury; both their spatial location and the electron energy spectra within them bear qualitative and quantitative resemblance to corresponding observations at the earth. In general, the magnetosphere of Mercury resembles to a marked degree a reduced version of that of the earth, there being no significant differences of structure revealed by the Mariner 10 observations. Quantities in the two magnetospheres are related by simple scaling laws. The size of Mercury relative to its magnetosphere precludes, however, the existence of stably trapped particle belts and of inner magnetosphere (L≲8 at the earth) phenomena generally. It is also expected that gradient-curvature drift and loss cone effects should be relatively more important at Mercury, but no observations bearing on these points were obtained. Due to the limited shielding provided by its relatively weak magnetic dipole moment, the surface of Mercury is everywhere subject to bombardmemt by cosmic rays and solar energetic particles with energies greater than 1 MeV/nucleon. The region of potential precipitation by magnetospheric plasma particles is estimated to be a ring, roughly similar to the terrestrial auroral oval but broader and lying at lower latitudes, 50--57¿ on the noon meridian and 25--35¿ at midnight. The observed magnetospheric plasma densities are generally too high to be accounted for by any plausible ionospheric source, and a solar wind origin is presumed.