Chilinidae is a family endemic to South America, ranging from the Tropic of Capricorn to Cape Horn and the Falkland Islands, and includes 32 species. However, there are few population studies on the Chilinidae. We study aspects of the ecology of an endemic species, Chilina megastoma Hylton Scott, 1958, from the Arrechea Falls in the Iguazú National Park, Argentina, such as density and individual annual growth trends. Nine samplings were carried out between December 2003 and December 2005, using two transects that crossed the waterfall. Individual annual growth rate was analyzed according to length, following von Bertalanffy's model. Six cohorts were identified, some in the same climatic season but successive years (two in winter and two in summer). The winter and autumn cohorts reached 85% of their last whorl length in the first year. Compared to other families of gastropods from subtropical climates, these populations have several recruitment events per year, but never in winter.