Ruppia maritima has the potential to become a model laboratory organism for studies with submerged aquatic vascular plants. The present study demonstrated that algae-free R. maritima grew well in a defined medium without sediment. Growth was a linear response to photon flux density over the range of light tested. Vitamins may be a necessary addition in artificial seawater. Trace metals caused little or no increase in growth during short-term (3 wk) growth studies, but appear to be required for long-term cultivation. Iron also caused no increase in growth, at the concentrations tested, but plants were greener in 1.46 micrometers M iron. A nitrate concentration of 110 micrometers M and a phosphate concentration of 2.3 micrometers M were sufficient for maximum growth. However, 4.5 micrometers M phosphate eliminated occasional CaCO3 precipitation in stock cultures. Critical tissue nitrogen content was between 2.5 and 3.0%, and critical phosphorus content between 0.25 and 0.35%. A comparison with field data suggests that R. maritima was deficient in both nitrogen and phosphorus during much of the summer.