Ontogenetic changes in the vertical distribution of a cohort of juvenile bay scallops, Argopecten irradians, on eelgrass, Zostera marina, were followed throughout the summer and early fall in two Long Island embayments (New York, USA). Despite site-specific differences in eelgrass height and density, more than 95% of post-settlement scallops remained attached above the bottom until they reached a shell height of about 11 mm. Over a 5-week period, scallops gradually relocated until, at a mean size of 31 mm, all occurred on the bottom. The decline in percent attachment coincided with a 5-fold increase (from 16 to 84 μmoles min-1 g muscle dry wt-1) in the activity of octopine dehydrogenase (proposed here as an index of the scallops' capacity for burst swimming activity), and in maximum rate of increase in the shell aspect ratio. While attached to eelgrass, scallops were nonuniformly distributed, with greatest concentration at mid-canopy. Following disturbance, they rapidly regained above-ground position, attaining asymptotic heights within 3-10 h. This and prior studies suggest that the climbing behavior of the bay scallop is an adaptive response to high predation pressure at small sizes. Enhanced scope for activity (predator avoidance) may enhance survival of scallops at intermediate sizes, when they become too heavy to maintain elevation but have not yet attained effective refuge in size.