Interstitial water analyses from numerous piston, gravity, and Alvin push cores show that fluid flow at the Costa Rica Accretionary Prism is spatially limited and can only be detected by visually directed cores in zones of biogenic activity. Most of the sites cored show evidence for normal diagenetic processes in rapidly deposited, organic carbon-rich sediments, with little evidence for fluid advection. However, in visually directed Alvin push cores, obtained from black sulfidic sediments characterized by the presence of Calyptogena clams or tubeworms, evidence for fluids advected upward from greater depth horizons is shown. These zones are associated with a large mudvolcano on the prism and with the zoes of Out Of Sequence Thrusts. As changes from sea water concentrations are still relatively small, substantial mixing with sea water must have occurred during this upward fluid movement.