Styles of deformation and tectonic responses resulting from the convergence of oceanic and continental plates are strongly dependent on fluids in the sediments. We estimate the volume of fluids and sediment underthrust beneath the toe of an accretionary wedge and describe evidence of fluid migration observed in seismic data in the Middle America Trench near Costa Rica. Reduction in normal incident travel time of the underthrust oceanic plate section arcward of the deformation front may be related to fluid expulsion. Within 4 km of the deformation front, where overburden is <800 m, more than half of the total water in the pore spaces may have been expelled. The rate of fluid transfer from the underthrust section is highest in the zone 0–4 km arcward of the deformation front. The clarity of the reflection associated with the décollement is exceptional and shows a phase reversal, relative to the seafloor. This high-amplitude phase-reversed reflection ends about 4 km arcward of the deformation front, suggesting a rapid reduction in the density and/or velocity contrast between the dewatering underthrust section and overlying offscraped sediments at this position. Fluid migration within the wedge produces mud volcanoes or ridges in the midslope region having relief of about 50 m and extent of perhaps 500 m×1000 m. A reflection traced beneath a seafloor mud volcano crosses through the accretionary wedge/slope cover boundary and may mark a fluid conduit. Locally, high-amplitude reflections associated with the base of slope cover are coincident with closure of structure and indicate local traps to upward organized or diffusive fluid flow. Some fluids appear to migrate along pathways marked by reflections which extend deep within the accretionary wedge but not always extending into the underthrust section. Near the trench where fluid expulsion rates are highest, water may exit to the seafloor seaward of the downslope pinch-out of the slope section.