Previous investigations of collisionless magnetic reconnection in a standard Harris neutral sheet configuration have demonstrated the importance of the Hall term for producing near-Alfv¿nic rates of reconnection and the existence of a very thin (~c/ωpe) electron current layer and sharp density/pressure gradients on c/ωpi scales. The present work uses three-dimensional (3-D) particle-in-cell simulations with an open geometry to investigate the changes in the reconnection physics produced by a guide field component B0y of the magnetic field. With B0y ≲ B0, the nonlinear reconnection rate is not substantially modified from that for the Harris case. The properties of the reconnection fields and particle dynamics, however, are strongly altered. The familiar quadrupole By pattern is replaced by an enhancement of ∣By∣ between the separatrices. The enhanced parallel electric field and parallel electron velocity are confined to one pair of separatrix arms (which are positively charged), while the electron current peaks on the other pair (which are negatively charged). The ion outflow along the current sheet polarizes the separatrices, thereby creating large components of the in-plane electric field. The electrons are accelerated to form a beam structure with parallel speed limited by the electron Alfv¿n speed. The beam-dominated electron distribution produces some y-dependent structures in E∥. For B0y ≫ B0, the reconnection rate is reduced by a factor of 2--3, and the parallel fields and velocities are somewhat smaller; the Hall current produced perturbations in By are considerably reduced.