【Background】[JP2]The flea beetle, Agasicles hygrophila, is an introduced natural enemy of alligator weed Alternanthera philoxeroides. The population density of A.hygrophila would effect the control effect on the alligator weed. It’s important to determine the population dynamics and dispersal rate of A.hygrophila when making feasible release strategy for the control A.philoxeroides.【Method】In this study, we released A.hygrophila at different densities in a marsh land in May and investigated their population dynamics from then on till December 2008.【Result】There were two obvious seasonal activity peaks during June~July and October~November, The value of these peaks was positively correlated with the initial releasing densities. Eighteen days after release, A.hygrophila density at 6 m distance away from the release site in dry land was higher than in either marsh land or in paddy land.【Conclusion and significance】The best release period for A.hygrophila was from May to June in early spring and the suitable release strategy, including release site density and population density should be determined based on its dispersal ability at different environmental conditions.[JP]