According to the sel"sh herd hypothesis, animals can decrease predation risk by moving toward one another if the predator can appear anywhere and will attack the nearest target. Previous studies have shown that aggregations can form using simple movement rules designed to decrease each animal's Domain of Danger. However, if the predator attacks from outside the group's perimeter, these simple movement rules might not lead to aggregation. To test whether simple sel"sh movement rules would decrease predation risk for those situations when the predator attacks from outside the #ock perimeter, we constructed a computer model that allowed #ocks of 75 simulated "ddler crabs to react to one another, and to a predator attacking from 7 m away. We attacked simulated crab #ocks with predators of di!erent sizes and attack speeds, and computed relative predation risk after 120 time steps. Final trajectories showed #ight toward the center of the #ock, but curving away from the predator. Path curvature depended on the predator's size and approach speed. The average crab experienced a greater decrease in predation risk when the predator was small or slow moving. Regardless of the predator's size and speed, however, predation risk always decreased as long as crabs took their #ock-mates into account. We conclude that, even when #ight away from an external predator occurs, the sel"sh avoidance of danger can lead to aggregation.