High abundance of invasive red-eared slider (RES) turtles (Trachemys scripta elegans) in Japan has been a serious issue in freshwater ecosystems. Information on reproductive parameters, such as sex ratios, of RES turtles is needed to effectively control the invasive species. We conducted a long-term field study on RES turtles at two study sites: the Taisho River (a shallow river) and Teradaike Pond (an irrigation reservoir that was irregularly drained to maintain the embankment and water quality) in western Japan. Using trapping and muddling methods, we captured RES turtles and checked for sex, carapace length, and body weight. We also examined whether the population of RES turtles could be reduced by continuously removing the turtles captured at each study site. The sex ratio (male/both sexes) of RES turtles that could be sexed by external morphology (presumably adult turtles) was 0.27 at the Taisho River and 0.26 at Teradaike Pond. At both sites, these sex ratios were almost the same as those of RES turtles that could be sexed only by necropsy after euthanasia (presumably juvenile turtles). These sex ratios were markedly biased toward females and consistent between the adult and the juvenile turtles, thus suggesting that more females had been produced than males. At the Taisho River, a total of 1,366 RES turtles were captured and eliminated over more than ten years. Nevertheless, the annual catch did not tend to decrease and only an effect of preventing population growth was observed. At Teradaike Pond, a total of 1,831 RES turtles were captured over nine years. Nearly half of them were caught for the initial two years, during which the pond was drained. Thereafter, the annual catch of RES turtles clearly decreased. These results suggest that it is difficult to control RES turtles established in rivers but that it is feasible to effectively control RES turtles in enclosed small water bodies, such as irrigation reservoirs, by intensively capturing and eliminating the turtles during the drainage period.