In mid-Autumn 1983, the outbreak for the hairworm to be discharged through a tap with drinking water occured within a certain district of Kanagawa prefecture in Japan.After microscopic examination this worm was recognized as a kind of parasite belonging to Gordioidea, Nematomorpha.The specific characters of this worm were as follows.1) This worm, preserved in 70% alcohol, was darkbrown in color and about 280mm long by 1.2 mm broad at the thickest portion.2) The body surface was covered with a smooth cuticula assumed the crossed light fibrous lines.3) At a short distance anterior to the point of bifurcation were found a crescent-shaped ridge and the round cloacal opening.By the chazacteristic signs mentioned above the specimenwas identified asGordiussp. (Family; Gordiidae) .It appears to be the first report that the hairworm was detected from drinking water in Japan. Regarding the same case of the hairworm, other 4 cases have been recorded up to now in Japan and all worms were also identified as Gordiu ssp..This article describes the biology of the hairworms, the cases found in drinking water, the parasitological and clinical aspects of the human infection, and the preventive measures inremov ing them.