Sostra Engineering bought the shares through a single deal concluded on the floor of the Bulgaria Stock Exchange's (BSE) regulated market on December 22, Stara Planina Hold said in a bourse filing.
The acquired shares represent 0.019% interest in Stara Planina Hold.
Sostra Engineering held a stake of under 5% in Stara Planina Hold at the end of June.
As at 15:40 CET on Wednesday, shares in Stara Planina Hold traded 2.88% higher at 5.35 levs on the BSE.
Sostra Engineering is 100%-owned by private individual Sasha Uzunova, who is related to the chairman of Stara Planina Hold's board of directors - Evgeniy Uzunov.
(1 euro = 1.95583 levs)