In large pot of boiling water, cook whole spaghetti squash until tender when pierced with skewer, about 30 minutes (some varieties of spaghetti squash may take longer).
In small skillet, melt margarine over medium-low heat; add garlic and cook until tender, about 1 minute.
Drain squash and cut in half crosswise.
Scoop out seeds.
Run tines of fork lengthwise over squash to loosen spaghetti like strands; scoop out strands into baking dish or serving bowl.
Add garlic mixture, parsley, yogurt (if using), and pepper to taste; toss to mix.