One thousand Lymnaea bulimoides were marked and released at two points in a 91 x 0.61 x 0.61 m drainage ditch eight kilometers south-west of Corvallis, Oregon, in December, 1968. They were observed from December, 1968 to March, 1969. A rheotaxis to movement against water flow was observed. During the first 24 hours an average distance of 0.91 m was recorded for snails moving upstream and 0.31 m for those moving downstream. Between the third and seventh week, an average distance of 1.52 m both upstream and downstream was recorded. By the tenth week, L. bulimoides had moved an average distance of 3.35 m upstream and 1.83 m downstream. Possible reasons for these movements are discussed.