This paper embodies the results of observations on the plankton in eight fish ponds in Heliekou, Wuxi ahi and three fish ponds in Wuli hu fish farm during the growing,in 19TT. samples were collected at 5--7 days intervals from four main ponds and at semi-month intervals from seven ordixsary ponds. The biomass of phytoplankton determined by method of calculating cell volume genera,Ily varies between 2fl--10DmgJL, in which the flagellate algae were predominar ted and Cyanophyte was rare.The average biomass of zaoplanh-ton is about the iJ4 to 1/3 that of phytaplankton. Rotifers are dominate.The fish farmers ju, the quality of water by observing of water colour. The colour of tine fertile water is mainly caused by the water blooming of four dominate planktons: Ganyostamum bloom, Cryptomonas bloom, Gymnodinium bloom end Chlorococus bloom. According to the experience of the fish--farmer, the brawn green water which. caused by Gonyaatomum bloom is the best fertile water, the brown water caused by Cryptomonas bloom and the Green water caused. by Chlorococus bloom are the medium ferta7e water，while the socalled changing water which caused the Gym- nodinium bloom is。intermediat form which either may change to be good water or may change to be bad water. Based on a rough osculation the production of the silver and hiad carp about so--so comes from the plankton energy in the different timers.