AbstractThe economic potential of the Mekong Delta is largely unrealized because of the harmful effects of uncontrolled flows of surface water which occur generally during the period August‐October. Interest in the Delta by various government agencies has resulted in preliminary plans for redistributing surface waters to control flooding, facilitate drainage, provide for irrigation, and prevent sea‐water encroachment. Current estimates indicate that construction costs in excess of a billion dollars would be required to develop an initial 2.1 million hectares (5.2 million acres) of the Delta to maximum economic potential; these costs do not cover those for upstream projects upon which the Delta developments depend.The Mekong Delta is underlain by an upper section of Recent alluvium, and a lower section of older alluvium. The older alluvium contains a permeable artesian zone called the 100‐meter aquifer, which is the most productive groundwater reservoir in Viet Nam. Tested well capacities range from about 145 to 635 gallons per minute (gpm); more efficiently designed wells should produce in the range of 500 to 1,000 gpm from this aquifer. Part of the 100‐meter aquifer is intruded by sea water.Current data permit no more than speculation with regard to storage, recharge, and flow in the 100‐meter aquifer. Storage is estimated to be approximately 30 million acre‐feet in the lower Delta, where the total dry season irrigation requirement is about 1.2 million acre‐feet. Piezometric levels in the Delta wells may be due merely to sea‐water pressure gradients acting across a horizontal saline‐fresh‐water interface; they do not necessarily imply ground‐water flow or an area of recharge.A major inconsistency exists in the relation of certain reported piezometric levels and corresponding depths to well intakes assuming conditions of either hydrostatic or hydrodynamic equilibrium, and it is necessary to postulate special circumstances to account for this. The data also indicate conditions conducive to subsidence, although no evidence for subsidence has yet been observed.The most feasible plan for development of the Mekong Delta may involve the conjunctive use of surface water and ground water of the 100‐meter aquifer, even though induced recharge and a ground‐water barrier against sea‐water intrusion might be necessary. Storage capacity is adequate, but more information is needed on recharge and total obtainable well capacities for the 100‐meter aquifer and on the possibilities for occurrence and control of subsidence in theeventof widesp