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Juvenile bay scallops Argopecten irradians commonly attach to shoots of eelgrass Zostera marina using byssal threads. Although this behavior has long been recognized, its adaptive value is poorly understood. This study examined (1) the size-specif~c nature of scallop attachment on eelgrass, and (2) the possible role of vertical attachment in providing refuge from benthic predators. Laboratory experiments using artificial eelgrass showed strong, inverse relationships between scallop size (over the range 6 to 20 mm) and several measures of attachment performance (percent attachment, rate of attachment, and height-above-bottom attained). Field experiments in which 10 to 15 mm scallops were tethered to natural eelgrass in Lake Montauk, Long Island, New York (USA), demonstrated a dramatic, highly significant enhancement of scallop survival at greater heights of attachment. Scallops tethered at 20 to 35 cm above bottom experienced > 59 O/ O survival over 4 d, compared to < l 1 O/O sunrival near the sediment surface. A similar pattern was observed in laboratory tethering experiments using transplanted natural eelgrass and 3 crab predators common in mid-Atlantic embayments: Carcinus maenas, Libinia dubia, and Dyspanopeus sayi. The refuge value of vertical attachment was found, however, to b e less with D. sayi than with the other predators tested, since individuals of t h s species climbed eelgrass to feed on scallops in the upper canopy. These data demonstrate that vertical attachment on eelgrass functions as an effective predator-avoidance mechanism for juvenile bay scallops by placing them out of reach of many benthic predators. It has also been previously shown that scallop predatory risk decreases with scallop size, with a partial size refuge attained at about 40 mm. It is therefore suggested that scallops undergo a n ontogenetic shift from a spatial to a size refuge from predation a s they move from the eelgrass canopy to the sediment surface over the course of their post-settlement life history. We hypothesize that this shift may result in a critical window of high predatory risk between ca 15 and 40 mm, when scallops are expected to be most susceptible to benthic predators.