This paper examined the increase and decrease of organizational populations in Korean firms. The results showed as follows. First, there was a positive association between the size of the urban area and organizational increasing rate. Second, the greater the organizational population, the higher the increasing rate in that population. Third, the negative impact of wage rate and energy cost was expected, but the hypothesis was to be rejected. Fourth, political turbulence and economic cycles appear to have little or no effect on the change of organizational populations over time. Fifth, from an ecological view, there are three major age groups in an organizational population : prereproductive, reproductive, and postreproductive. Under these conditions mining population appeared to be a bell or an urn-shaped age structure, and manufacturing population appeared to be a pyramid-shaped age structure. For this study, it is important to stress the need for longitudinal research. Therefore, from now on, it is necessary to collect and arrange the data throughly and continually.