A method is described for the quantitative determination of deoxyribonucleic acid (DNA) by measurement of the fluorescence of the complex formed by incubating samples containing DNA with diaminobenzoic acid dihydrochloride. The sensitivity and range (from 0.2 to 40‐µg of DNA) of this method permit DNA determination on seawater samples of 1 liter or less from the euphotic zone and on 1 to 4 liters from deep samples. The concentration of DNA has been measured at various stations in the Atlantic and Pacific oceans, together with measurements on chlorophyll, zooplankton, and organic carbon and nitrogen of the particulate material. These data are discussed in relation to the possible use of DNA as a biomass indicator.The DNA content of laboratory cultured cells of Skeletonema costatum, Dunaliella tertiolecta, and Monochrysis lutheri have been determined relative to the cellular contents of organic carbon, nitrogen, and chlorophyll. When expressed as a percentage of the total cellular dry weight, the DNA contents of these three marine phytoplankton species were 0.43, 0.66, and 0.40%, respectively.