In a small bowl mix together balsamic, olive oil, lemon juice, garlic, pepper and salt.
Snap ends off asparagus, wash to remove soil and debris and place into a freezer bag. Pour the vinegar mixture over the asparagus and let sit for 30 minutes.
Place on hot grill and grill 5 minutes each side or until stalks are tender.
To help prevent stalks from falling through the grates of the grill, bunch 5 stalks side by side and stick a toothpick through the bottom and top to hold them in place.