The stomach of each of four individuals contained one _Rana nigromaculata_. The stomach of another individual contained a _R. nigromaculata_ and remains of a carabid beetle, whereas another contained three small, partially-digested frogs that appeared to be _Hyla arborea_. P. M. Youngman reported to us that he found a snake of this species that was attempting to swallow a toad, _Bufo bufo gargarizans_. One of the small individuals from Cheju Do was being eaten by a _Zamenis spinalis_ when found. One specimen was parasitized by three nematodes, _Kalicephalus natricis_ (see Olsen, 1957:208). Two females of this oviparous species (lengths of body, 680 and 700) collected on May 14 contained nine eggs (18 mm. long), and 13 eggs (15 mm.) respectively; a third (length of body, 610) obtained on June 26 contained 10 eggs that were approximately 18 mm. long. A female (UMMZ 113458, length of body, 710), which was captured on July 10 and kept alive in captivity, laid 11 eggs on August 12 between 9 and 10 in the morning. The weight of nine of these eggs averaged 3.32 (3.0-3.6) grams; the last two eggs deposited were small and weighed only 1.3 and 1.4 grams. The eggs were incubated unsuccessfully. One that was opened on September 14 and another opened on September 26 contained young easily recognized as of this species. In captivity the parent snake underwent ecdysis on about July 20 and again on August 26. Our largest female and largest male have respective total lengths of 1013 (840 + 173) and 740 (575 + 165). Our smallest specimens, captured on September 9, measured 215 and 230 mm. in length of body, and probably represent young of the year. The snake found in hibernation on April 5 measured 275 in length of body. The ventrals of 11 males averaged 161.3 (158-171) and those of 14 females, 165.1 (160-170); subcaudals of eight males averaged 69.6 (66-74) and those of 14 females, 61.5 (52-73). Males seem to have small scales in the anal region that are more strongly keeled than scales elsewhere on the body (the scales catch on finger tips when rubbed in a posteroanterior direction), but males lack small tubercles on the upper and lateral parts of the head as mentioned by Maslin (1950:433). The comments of the same author (_op. cit._:434) concerning integumental poison glands in the nuchal region of this species are of interest in view of several reports that we received of swollen extremities resulting from handling snakes of this species. In using the generic names _Rhabdophis_ and _Amphiesma_ for species formerly placed in the genus _Natrix_, we follow Malnate (1960), who divided _Natrix (auct.)_ into five distinct genera. =Amphiesma vibakari ruthveni= (Van Denburgh) _Natrix vibakari ruthveni_ Van Denburgh, Proc. California Acad. Sci., ser. 4, 13(2):3, July 26, 1923 (type locality, Pusan, Korea).