We analyze a dense grid of seismic normal incidence and wide-angle data collected over an active back arc spreading center in the Lau Basin of the southwest Pacific. Our survey area covers the whole of a 35-km-long morphological segment and a small overlapping spreading center. A bright reflector, which is coincident with a velocity inversion at a depth of 3.2¿0.2 km below the seafloor, is observed on every one of the 40 across-axis profiles. We interpret this reflector as being the roof of a crustal magma chamber. The widest magma chamber reflector occurs beneath the overlapping spreading center, where it extends up to 4 km, being imaged beneath both ridges and, in places, beneath the overlap basin. Elsewhere the width of the reflector varies between 0.6 and 2.3¿0.4 km. The narrowest reflectors are observed beneath deviations (devals) of the ridge axis. In addition to being observed on each of the kilometer-spaced across-axis profiles the reflector is also seen as a continuous event for at least 10 km on a profile along the ridge axis. The reflection coefficient of the event is beint -0.34 and -0.65. This requires the melt at the top of the chamber to have a P wave velocity less than 2.7 km/s if the interface is planar or less than 3.8 km/s if the interface is layered.