Three-dimensional magnetic reconnection is studied using magnetohydrodynamic simulations. The initial configuration is based on the two-dimensional Harris neutral sheet model that lies in the xz plane and is extended in the y direction. Localized anomalous resistivity is applied to the central region, and the subsequent evolution of spontaneous magnetic reconnection is observed. Special attention is given to the results with a finite By superimposed on the Harris model. Significant changes are seen in the reconnection morphology, as the By component causes asymmetries. The reconnected field lines are skewed, and the plasma flows, shock structures, and current flows show the corresponding peculiar asymmetries. The plasma sheet is also seen twisted. A broader region is affected by magnetic reconnection as By increases, and it is seen that energy conversion over the whole simulation domain is more significant when By is larger, unless By is the dominant component of the magnetic field. The field-aligned component of the current, which initially exists because of the finite By component, is enhanced off the central plane when reconnection develops, while it is reduced on the central plane. The spatial scale of resistivity affects the reconnection rate as in previous studies of By=0, yielding a small energy conversion for a very localized model resistivity. ¿ 2000 American Geophysical Union