The seaweed Ulva lactuca L. was spray cultured by mariculture effluents in a mattress-like layer, held in air on slanted boards by plastic netting. Air-agitated seaweed suspension tanks were the reference. Growth rate, yield, and ammonia-N removal rate were 11.8% Æ d)1, 171 g fresh weight (fwt) Æ m)2 Æ d)1, and 5 g N Æ m)2 Æ d)1, respectively, by the spray-cultured U. lactuca, and 16.9% Æ d)1, 283 g fwt Æ m)2 Æ d)1, and 7 g N Æ m)2 Æ d)1, respectively, by the tank U. lactuca. Biomass protein content was similar in both treatments. Dissolved oxygen in the fishpond effluent water was raised by >3 mg Æ L)1 and pH by up to half a unit, upon passage through both culture systems. The data suggest that spray-irrigation culture of U. lactuca in this simple green-mattress-like system supplies the seaweed all it needs to grow and biofilter at rates close to those in standard air-agitated tank culture.