The excretion of photoassimilated carbon was determined for 22 species of unicellular marine algae in culture during periods of log‐phase growth and for some natural marine phytoplankton populations from Vineyard Sound and the Gulf of Maine. Carbon 14 tracers gave some information about the composition of the excreted material. Most of the algae excreted 3–6% of their photoassimilated carbon during logarithmic growth. A few species excreted as much as 10–25% under the same conditions. Algae subjected to two different light intensities, 3,000 lux and 25,000 lux, varied less than 30% in their relative rates of excretion. However, algae exposed to direct sunlight (100,000–120,000 lux) had very high excretion rates, possibly resulting from damage to the cells by photooxidation. Glycolic acid formed 9–38% of the total carbon excretion in Olisthodiscus sp., Chaetoceros pelagicus, Chlorococcum sp. and Skeletonema costatum. Lower relative amounts were excreted by most of the species studied. Skeletonena costatum excreted considerably more glycolic acid at light intensities below 15,000 lux than above. This is contrary to the findings of other investigators using the freshwater alga Chlorella pyrenoidosa. The amount of carbon excreted as protein ranged from 0.2 to 5.9%, and that excreted as chloroform‐soluble material ranged from 2.8 to 10.3% of the total. After electrodialysis, 12–34% of the compounds were recovered in the anion fraction. Considerable quantities of amino acids and peptides were also detected as excretory products of many of the algae. A few species excreted a single substance almost exclusively. Thus, Chlorella sp. excreted mainly proline, Dunaliella tertiolecta mainly glycerol, and Olisthodiscus sp. mainly mannitol. Apparently healthy populations of natural phytoplankton excreted 4–16% of their photoassimilated carbon. However, 17–38% was excreted by a sample taken at the end of a diatom bloom, when a large number of empty frustules was present.

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SUMMARY Clones of Phaeocystis were isolated from winter surface waters off Woods Hole, Massachusetts, and from the tropical Atlantic near Surinam, South America. Northern strains survived only up to 14 C, while the tropical strain survived only as low as 17 C. Colony shapes of the northern and tropical clones differed somewhat, but the motile and non‐motile single cells of both strains seemed identical in the light microscope. By current taxonomic criteria both strains belong to the species P. poucheti (Hariot) Lagerheim. When growing in the form of colonies, both strains excreted 16–64% of their photoassimilated carbon into the medium, mainly as carbohydrates of varying molecular weights. However, cultures predominantly in the form of single cells released only about 3% of their photoassimilated carbon. The qualitative composition of the carbohydrates released is similar for the 2 strains, consisting of some 8 sugars or sugar derivatives with glucose, mannose, and rhamnose as the dominant components. The production of acrylic acid was confirmed. We estimate that as much as 7 μg/liter of acrylic acid, and at least 0.3 mg/liter of polysaccharides can be liberated in a Phaeocystis bloom.