1. The population dynamics of the pine needle gall midge, Thecodiplosis japonensis Uchida et Inouye, was studied during the period from 1975 to 1982 in a young stand of Pinus thunbergii at Kamigamo Experimental Forest Station of Kyoto University, Kyoto. 2. The population density was estimated for six developmental stages, and the mortalities operating on T. japonensis were described. The declines in the population numbers were considered to be caused mainly by abiotic factors rather than biotic ones such as predation, competition and so on. 3. The natural population of T. japonensis was depressed at very Low density and well regulated. Life table analysis for five generations of population data indicated that the key factor was failure to realize maximum fecundity. Population regulation by density dependent mechanisms could not be detected from the census data.