Rinse mushrooms; pat dry.
Place in center of an 18-inch square of heavy foil.
Fold up foil to surround but not enclose mushrooms.
Combine Worcestershire sauce, margarine, garlic salt, and pepper; drizzle over mushrooms.
Place open packet in center of the cooking grill.
Grill 20 to 30 minutes or till mushrooms are tender.
Remove from grill.
Spoon juices over mushrooms.
Serve mushrooms with a slotted spoon.