The opposition centre-right Union of Democratic Forces (UDF) elected on Monday Emil Kabaivanov as its chairman.
Kabaivanov was backed by 3,941 votes, equivalent to 51.90% of the total, against 3,652 for his opponent Vanyo Sharkov, the state-run Bulgarian News Agency, BTA, reported quoting preliminary results made available by UDF's election commission chairman.
Some 85% of UDF members cast a ballot in the second round.
The extraordinary election was held after Martin Dimitrov resigned as party leader in mid-May over the UDF's decision to leave the Blue Coalition where it used to partner with Democrats for Strong Bulgaria led by former prime minister Ivan Kostov.
Five candidates ran for party leader.