An electrometric system for determination of carbonic anhydrase activity was constructed. Enzyme activity was assayed in homogenates of marine macroscopic Chlorophyta, Rhodophyta, and Phaeophyta. Plants surveyed included Ulva expansa (Setchell) Setchell and Gardner, Codium fragile (Suringar) Hariot, Enteromorpha sp., Chaetomorpha torta (Farlow) McClatchie (Chlorophyta); Laurencia papillosa (Greville). Plocamium coccineum var. pacificum (Kylin) Dawson, Pterocladia capillacea (Gmelin) Bornet and Thuret. Gigartina armata J. Agardh (Rhodophyta); Eisenia arborea Areschoug and Macrocystis pyrifera (Linnaeus) C. A. Agardh (Phaeophyta). Activity was present in all algae; in the Phaeophyta this could be demonstrated only after dialysis. p-Chloromercuriphenylsulfonic acid (10(-4)m) decreased activity in 1 species, Plocamium; this inhibition could be almost completely overcome with the addition of 10(-3)m dithiothreitol. In 2 green and 2 red algae assayed for sensitivity to acetazolamide (Diamox), inhibition was complete at 10(-4)m concentration of inhibitor. Dithiothreitol at a concentration of 10(-3)m did not enhance activity in any of the homogenates, and was not necessary for enzyme expression.