Beta Upsilon Chi, Brothers Under Christ, BYX (pronounced "bux") is the largest Christian social fraternity in the United States. Since its founding in 1985, BYX has spread across the country.

The fraternity exists to "establish brotherhood and unity among college men based on the common bond of Jesus Christ." The fraternity''s founding Scripture is Psalm 133:1, "Behold how good and pleasant it is for brothers to dwell in unity!"