The use of heptachlor, a cyclodiene-type insecticide, has been banned since the 1970s because of its carcinogenic potential. However, its metabolite, heptachlor exo-epoxide (HEPX), has still been detected in the fruit of cucurbits produced in some areas. It is important to reduce the daily intake of HEPX contained in food. To address this issue, the effects of carbonaceous adsorbents on the uptake of HEPX from the soil by Cucurbita maxima Dutch. (winter squash) were investigated. Amorphous organic carbons, such as peat moss and wood chips, did not affect the concentration of HEPX in the soil solution or the amount present in the shoots. In contrast, relatively condensed carbon, such as activated carbons, decreased the concentration of HEPX in the soil solution and in the shoots. The uptake amount in the shoot was closely correlated with the concentration of HEPX in the soil solution, which suggests that its uptake by C. maxima depends on its concentration in the soil solution. Activated carbons did not affect the growth of C. maxima. Therefore, the application of activated carbon to soil contaminated with HEPX appears to be an effective method of reducing the uptake of HEPX by C. maxima.