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To determine whether California horn snails are more likely to be consumers or facilitators of Ulva expansa (Setch) S. & G. growth in estuaries, we conducted manip ulative expm.ments that evaluated algal growth and the movement of N between the water column, algal tissue, and, in the second experiment, sediments. Algae grew poor-4 in the absence of sediments, drawing on their own sequestered N supplies (3.5% of d? weight reduced to < 2 %) and N released by snails and by depleting inorganic N in the water column. There was no evidence of consumption when snail densities ranged from 0 to 900,m-2 (0, 3, 6, and 9 per aquarium), as algal growth was similar for all snail densities, and snail lengths did not increase during the 21-d expm'ment. when sediment was provided, N was depleted in the sediment and enhanced in the algal tissue. As in thejirst experiment, the water column was depleted of inorganic N and enriched with organic N, mostly in the dissolved form. Because both snails and macroalgae often dominate the shallow waters of southern California's lagoons and estuaries, our evidence that the snails are primarily facilitators of algal growth (via transfer of N from sediments to the water column) suggests that snails may play an important role in both food web and N dynamics.