A mass culture of Tetraselmis suecica grown in seawater enriched with only inorganic nutrients and CO2 in a shallow outdoor flume containing foil arrays to effect systematic vertical mixing achieved average daily production rates of over 40 g ash-free dry wt (AFDW)/m2 over periods as long as one month when grown on a three-day dilution cycle. Photosynthetic efficiencies associated with these high production rates averaged 8–11% based on visible irradiance. Operation of the system in a one-, two-, or four-day dilution cycle resulted in lower photosynthetic efficiencies of 6–7%. A remarkable feature of the three-day dilution cycle results was the fact that production on the third day after dilution averaged 60–70 g AFDW/m2, and corresponding photosynthetic efficiencies averaged 13–19%. The high production rates and photosynthetic efficiencies achieved on the third day after dilution may have reflected the nonequilibrium nature of the production cycle and, in particular, the fact that the adaptation of the cells to changing light condition lagged behind light condition in the culture.