The objective of the Geospace Environment Modeling (GEM) magnetic reconnection challenge is to understand the collisionless physics that controls the rate of magnetic reconnection in a two-dimensional configuration. The challenge involves investigating a standard model problem based on a simple Harris sheet configuration by means of a variety of physical models in order to isolate the essential physics. In the present work the challenge problem is modeled using an electromagnetic particle-in-cell code in which full particle dynamics are retained for both electrons and ions and Maxwell's equations are solved without approximation. The timescale for reconnection is of the order of 10 &OHgr;i-1 (where &OHgr;i is the ion cyclotron frequency based on the asymptotic field B0), and the corresponding reconnection electric field is (c/vA)Ey/B0~0.24. The diffusion region near the neutral line is observed to develop a multiscale structure based on the electron and ion inertial lengths c/&ohgr;pe and c/&ohgr;pi. The difference between the ion and electron dynamics in the diffusion region gives rise to in-plane (Hall) currents which produce an out-of-plane By field with a quadrupolar structure. In the diffusion region the magnetic field is no longer frozen-in to the electrons; the inductive Ey field is supported primarily by the off-diagonal electron pressure terms in the generalized Ohm's law. The reconnection rate is found to be insensitive to electron inertia effects and to the presence of a moderate out-of-plane initial field component B0y≲B0. The results are consistent with the theory that the reconnection rate is independent of the mechanism which breaks the frozen-in condition and is controlled by dynamics at length scales much greater than the electron dissipation region. ¿ 2001 American Geophysical Union