The dioecious snail Cerithidea rhizophorarum (Adams, 1855) is distributed along the coasts of the western Pacific up to the Tohoku district, northern Honshu, Japan. It inhabits reed grasslands and mangrove forests with Kandelia candel and Hibiscus hamabo trees on a mud flat located at the mouth of Atagogawa River in Kiire. Studies of mating and tree climbing behaviors of the species were conducted at this site from April 2000 to May 2003. Mating behavior was observed in July and August 2002. The time of commencement, termination, and duration were recorded for each copulation. The peak of matings during daytime was seen at 1 to 2, and 5 hours before the lowest tide and during nighttime, between 1 hour before and after lowest tide. However, mating rarely occurred on cloudy days. Climbing behavior was observed in an area of 100 square meters where only K. candel trees existed. The number of snails on trees was counted, and daily activity of the snails on trees was monitored in summer and winter, hourly throughout the day. The snails were mainly found on mud from spring to summer but frequently climbed up the tree at particular times during summer. Most individuals were on trees and motionless during winter.