Abstract: In order to investigate a internal relationship between emergence time and quantity changes of Craspedacusta and plankton,and to evaluate the water quality of Craspedacusta' habitat by Margalef and Menkinick diversity index,the water sample and Craspedacusta were collected in a small fire-fighting pond in Ningbo,Zhejiang Province from Nov,2005 to Dec,2005 and from Jan,2006 to Oct,2006.This kind of jellyfish was identified as Craspedacusta sowerbyi xinyangensis.The water in the fire-fighting pond mainly came from rain.The area of the pond was roughly 50 m2 and about 2-3 m in depth.While collecting,the water temperature was 5.7-30.2℃,pH 6.7-7.9,the color of the water is green,with a transparency 0.55-1.45 cm.Craspedacusta's life cycle underwent two alternating generations:sexual(medusa) and asexual(polyp).We gained Craspedacus(medusa) periodically by the microplankton net when it appeared(6-10 month) and collected the water sample monthly.In addition,the water sample was also collected every other month in 2007 when Craspedacus did not appeared for comparing the analysis.The main results were as follow:1) Totally 85 species remaining with phylum 8 and genera 53 were recorded.Chlorophyta was absolutely predominant in the species composition,representing 42.35% of the total phytoplankton.The dominant species of Cyanophyta and Chlorophyta mainly contained Chorella vulgaris,Merismopedia glauca and Microcystis incerta.There were 44 species in zooplankton,among which,rotifer and protozoan were absolutely predominant in the species composition,and accounting for 47.74% and 45.45% of the total of zooplankton respectively.The dominant species of zooplankton mainly contained Polyarthra sp.2) There was a abundance peak value of phytoplankton in November to 14 065.60×104 ind/L.The abundance of Cyanophyta was premier,representing 61.05% of the total;The abundances of zooplankton had two peak value in May and November,which had the maximum abundance of 5 580 ind/L in May,while the minimum abundance of 900 ind/L in July;The abundances of protozoan and rotifer accounted for 48.28% and 43.92% of the total respectively,and copepods for only 7.73%.The largest biomass was rotifer.3) The quantity of rotifer and copepods have all reached the peak value before the appearance of jellyfish;Abundance of rotifer was the minimum during the peak of jellyfish and rebounded again after the disappearance of jellyfish;Phytoplankton and zooplankton and Craspedacusta were in a dynamic equilibrium of the three.4) The water quality of pool belonged to the type of "little pollution-middle nutrition" before and middle appearance of Craspedacusta in 4-8 month,and developed to the type of "middle pollution-enrich nutrition" later appearance and after disappearance of Craspedacusta in 9-11 month.The main aim of this study was to probe into a internal relationship between population dynamics of Craspedacusta and quantity variations of plankton by qualitative and quantitative analysis in a small pool which is the Craspedacusta sowerbyi xinyangensis' habitat.It was also important to further do a series of ecological experiments such as studying the adaptability to the habitat conditions or selectivity of feeding habits.Finally,our study will help provide new approach for protection of Craspedacusta.