Procedures are presented for separate analyses of total dissolved phosphorus and total particulate phosphorus in natural waters. The method for both procedures involves drying a sample with magnesium sulfate and baking the residue at a high temperature to decompose organic phosphorus compounds. The residue is then treated with hydrochloric acid to hydrolyze polyphosphates and the orthophosphate is measured by the molybdate method. The method gives 100% recovery with refractory phosphorus compounds, is usable on undiluted samples with up to 18 µ mol of phosphorus per liter (6 µ g P per sample), and has a mid‐range precision of ±1%.