The low tidal range of the Mediterranean Sea and the high sediment load of the Rhone induce the formation of an important submarine delta. The 137 Cs inventory in sediment on a 480 km 2 area near the Rhone mouth reached 19.6 TBq in 1990. The spatial distribution of both sediment accumulation rates and 137 Cs concentrations in this area confirm the existence of a spatially well-delimited zone, i.e. the prodelta, where both parameters are the highest. Thus, 40% of the total amount of 137 Cs stored in this 480 km 2 area was found within the prodelta (30 km 2 ) in the close vicinity of the river mouth. This attests to the efficiency of the 137 Cs trapping by the prodelta sediments. A study on the part the different 137 Cs sources in the Mediterranean Sea play in contributing to this inventory has been carried out. These sources are (i) direct deposition from both global fallout and the Chernobyl accident, (ii) indirect inputs arising from the erosion of surface soils of the Rhone catchment area that were also contaminated by the previous fallout, (iii) liquid effluents from the nuclear industry into the Rhone waters. Assuming that direct deposition is evenly distributed over the study area (480 km 2 ) and that the particulate 137 Cs input from the Rhone is entirely trapped in this same area, these sources account for about 50% of the inventory. These hypotheses are unlikely in a coastal area subject to various physical disturbances but they are conservative enough to assess the lower limit of the nuclear liquid releases contribution to this inventory to be 50%.