The structure of the magnetotail plasma sheet-plasma lobe boundary is studied by observing the properties of tailward flowing O+ ion streams. These have been detected by the ISEE 2 plasma experiment inside the boundary during three time periods in April 1978: data from the ISEE 1 electron spectrometer are used to define the location of the boundary. The density of the O+ ion streams has a relative maximum, and their velocity has a relative minimum at the boundary. The xy drift velocity component across the local magnetic field reverses at the boundary, indicating a reversal of the Ez electric field component. These changes are consistent with the topology of the electric field lines in the tail as mapped from the ionosphere.