Groundwater contamination by pesticides or herbicides is getting one of the most serious environmental problems. This study focuses on water contamination by pentachloronitrobenzene (PCNB) on the northern foot of Mt. Asama, Gumma Prefecture, where cabbage patch is intensively distributed and a great amount of PCNB is applied. The area is composed of volcanic ashes, lavas and pyroclastic materials whose permeabilities are very large. Waters of river, groundwater and spring were sampled through April to October in 1987 and PCNB, PCA, PCTA, and other major elements were analyzed. Water quality is determined mainly by geologic formation and secondary by land use or agricultural activities. Groundwater as well as river water is contaminated by PCNB. The maximum values in river water and groundwater are 1.4 μg/l and 7.9 μg/l, respectively. Even spring water above cabbage patch area is also contaminated, which indicates PCNB is carried through atomosphere. PCA and PCTA, the principal degradation products of PCNB, are detected proportionally to PCNB and their rations to PCNB are relatively higher in September and October than in summer.