In the northern Gulf of St Lawrence, eastern Canada, the whelk Buccinum undatumoccasionally aggregates near the feeding asteroid Leptasterias polaristo obtain food. This is surprising considering that whelks are frequently preyed upon by L. polarisand have developed a violent escape response to this predator. Almost all whelks in aggregations near feeding asteroids are large. This study suggests that small whelks do not approach feeding L. polarisbecause they are more vulnerable to this predator. Predation experiments demonstrated that the vulnerability of whelks to L. polarisdecreases with increasing whelk size. Sampling in the field showed that 97% of whelks measuring more than 5 cm in shell length are found in the sediment zone, where L. polarisextracts large endobenthic bivalves, but whelks less than 5 cm are equally abundant in the rocky (49.5%) and sediment (50.5%) zones. Field experiments showed that whelks within 50 cm of large bivalves being ingested by L. polariswere larger than those within 50 cm of bivalves placed by themselves on the bottom. Finally, laboratory measurements showed that the influence of predator threat on the time whelks spent feeding decreased with whelk size, but the influence of feeding motivation on the time whelks spent feeding increased with whelk size. In the situation where motivation to feed was high and size-related differences in vulnerability were large, the impact of the predator decreased significantly with increasing whelk size. Whelks show threat-sensitive decision making adjusted to potential feeding benefits and vulnerability to predation.