ABSTRACT: Environmental and biological conditions of the intertidal zone vary according to tidal level. Monodonta labio (Gastropoda; trochidae) occurs over the whole range of the intertidal zone, but juveniles occur only in the mid intertidal zone. In thisstudy, vertical migration of this snail was investigated by mark-recapture techniques for 1 yr at Amakusa, Japan. Snails migrated vertically throughout the year, but varied with season and size. Generally, juvenile snails (&LT7 mm in shell width) did notactively migrate. Upward migration was conspicuous only in small snails (7 to 10 mm) in summer. Downward migration was greatest in the larger size classes. Thus, large snails (&GT=13 mm) gradually migrated downward to the lower zone. Seasonal fluctuationsin the vertical distribution pattern of M. labio could be explained by this vertical migration. Possible factors affecting this vertical migration and the adaptive significance of migration in the life history of M. labio arediscussed.