The Costa Rica hydrogeology operation was designed to replace CORK-II downhole instrument strings in holes drilled at Ocean Drilling Program Leg 205 Sites 1253 and 1255 (September–November 2002) off Costa Rica. The CORKs are instrumented in a fractured horizon in the oceanic section of the incoming plate (Site 1253) and in the décollement zone (Site 1255) to investigate fluid flow across the margin and its implications for the seismogenic zone and subduction factory. Operations during the cruise were successfully completed in less than 4 days, and the instrument strings were retrieved and replaced; only the lower osmotic fluid sampler OsmoSampler package from Site 1253 was lost. Because of the close proximity of the two Site 1253 OsmoSampler packages, this loss will have minimal impact on the data. All recovered instruments worked as expected, and the redeployed samplers and temperature loggers are configured for approximately 3.5 y duration. OsmoSampler fluids from each site were analyzed for ephemeral properties on board ship (salinity, pH, alkalinity, and ammonium) and subdivided for further postcruise studies. Postcruise analyses will provide a high-resolution time series of fluid chemistry and temperature over a 2 y period and, at Site 1255, fluid flow rates and relative flow directions, with implications for seismogenic zone activity, chemistry of the arc volcanoes, and solute fluxes into the ocean.