The exchange of phosphate between water and sediments of Doboy Sound, Georgia, was studied experimentally with freshly collected core samples and suspensions of surface sediment, using 32P as a tracer. The exchange consists of a two‐step ion exchange between clay minerals and water, plus an exchange between interstitial microorganisms and water. The exchange tends to maintain a concentration of phosphate in the water of one µmole of phosphate/liter. In undisturbed sediments the biological exchange is trivial, but in suspended sediments the biological exchange moves nearly as much phosphate as does the exchange with clay minerals. The rates of exchange and the exchange capacity of the sediments are large enough to be significant ecologically, maintaining phosphate at a level favorable for continued production of plant populations.