Population trends of the brackish water damselfly, Mortonagrion hirosei were studied for 4 years in the reed community artificially established for conservation of this endangered species. Because of difficulty with mark-and-recapture experiments on this small damselfly with weak wings in the large dense reed community, census counts using the line transect method were performed to estimate the population parameters. The reed rhizomes were transplanted in January of 2003. A few adults immigrated to the community in the flying season of this year, but they were restricted near the original habitat. The number of adults throughout the flying season was estimated at 1,000. In 2004, the population included both the immigrants from the original habitat and the emergences from the established habitat, and the total population was estimated at 10,000, and the daily density in peak flight season was 20% that in the original habitat. An estimated 23,000 individuals were found all over the established habitat in 2005. In 2006, the estimated number of adults in the established habitat was 45,600, and the population density increased almost equal to that in the original habitat. Therefore we can conclude that the damselfly had settled in the established habitat.