We examine the plausible existence of Neptune's plasmasphere and study the drift of particles inside it. Using the O8 magnetic field model <Connerney et al., 1991> and assuming a uniform solar wind convection electric field, the plasma convection time and refilling time are calculated in a Euler potential coordinate system <Ho et al., 1997>. The plasma density and refilling time at the equilibrium state are first calculated, and the location of the plasmapause is set to be where the refilling time and convection time are equal. The refilling time as a function of ion speed is then recalculated along field lines, and the plasma density and temperature are obtained by directly integrating the local ion distribution function over the range of speeds for which the refilling time is less than the convection time. The density calculated using this model shows sharp drop-offs at approximately 3.25 to 4.5 RN on the zero magnetic scalar potential surface, a boundary taken to be the plasmapause. Our calculated density compares fairly with the observed density along the Voyager trajectory within about 5 RN. Ion temperature is also calculated along the field line with results which indicate that high-speed tails of the distribution function might be needed to explain the high observed temperature measured along the Voyager 2 trajectory. Drift trajectories and speeds of 90¿ pitch angle particles inside the plasmapause are calculated. Particles of energy above tens of eV are gradient drift dominated, and the drift paths of this class of particles are essentially the minimum B contours that are similar to Acu¿a et al.'s <1993> calculations. Atmospheric precipitation of the J=0 particles may provide an explanation for the UV emissions, as an alternative to the monoprecipitation suggested by Paranicas and Cheng <1994>. Drifts of low-energy particles are strongly affected by the gravitational and centrifugal forces, and because of the largely tilted dipole and the large higher components of magnetic field, the resultant drift is nonaxisymmetric and quite complicated. ¿ 1998 American Geophysical Union