Put popcorn in a very large bowl. Butter two large cookie sheets.
Combine brown sugar, corn syrup, butter, and salt in medium saucepan. Bring to a boil, stirring constantly. Boil two minutes. Remove from heat; stir in baking soda. Immediately pour over popcorn. Toss until well blended.
Spread popcorn on buttered cookie sheets. Bake at 200 degrees for 1 hour, stirring every 15 minutes.