Two enclosure experiments were carried out from June to October in 1995 and June to August in 1996 to evaluate the effect of different types of monoculture or polyculture on water chemistry in enclosures .The results showed that concentration of total nitrogen and total phosphorus , chemical oxygen demand , and salinity increased before rainy season then dropped rapidly after rain . When only feed was adopted , the concentration of reactive phosphate and total phosphorus was relatively low and the concentration of nitrite , nitrate , ammonia and total nitrogen was high .When both feed and fertilizer was provided , the concentration of nitrite , nitrate , ammonia and total nitrogen was low and the concentration of reactive phosphate and total phosphorus was high .Concentration of reactive phosphate was usually higher in enclosures stocked with hybrid tilapia , Oreochromis mossambicus ×O .niloticus , while the concentration of ammonia was higher in enclosures stocked with Ruditapes philippinarum or Sinonvacula constricta .Chemical oxygen demand in present experiments was not as high as that usually reported in freshwater fishponds or seawater shrimp-ponds , and much lower in those enclosures stocked with bivalves .Present experiments suggest that water chemistry in experimental enclosures was mainly dependent on daily management activities such as feeding and fertilizing , and partly influenced by stocked shrimp , tilapia and bivalves at the same time .