This study employs Geotail plasma observations and numerical modeling to determine sources of the ions observed in the near-Earth magnetotail near midnight during a substorm. The growth phase has the low-latitude boundary layer as its most important source of ions at Geotail, but during the expansion phase the plasma mantle is dominant. The mantle distribution shows evidence of two distinct entry mechanisms: entry through a high-latitude reconnection region resulting in an accelerated component, and entry through open field lines traditionally identified with the mantle source. The two entry mechanisms are separated in time, with the high-latitude reconnection region disappearing prior to substorm onset. ¿ 1999 American Geophysical Union