Abstract: Polyculture of Chinese shrimp ( Penaeus chinensis ) with various biomass of Ulva ( Ulva pertusavar ) was conducted in ten aquaria of each 53cm @ 24cm @ 23cm from July to September in 1998. The results showed that Ulva could absorb N and P nutrients and improve water quality by decreasing the nitrogen and phosphorus loading and increase dissolved oxygen levels in the polyculture systems. The shrimp had total N and P utilization rates of 42. 7% - 212. 9% higher in the tanks with Ulva than those in the monoculture system. When the densities of shrimp and Ulva were optimal ( 320- 480 g#m- 3 and 400- 600 g#m- 3) , the oxygen produced by Ulva was suff icient for shrimp. s need without extra aeration.