...Exchange (PSE) in a single transaction on Monday, PSE data showed.
PSE data showed 11 million shares traded in a block trade at a price of 280 crowns per share in a transaction worth 3.08 billion crowns ($155.7 million).
The stock closed 1.84 percent higher at 292.6 crowns on Monday, beating the main PX index which lost 0.8 percent.
The bourse did not disclose the counterparties.
Poland's PKN Orlen holds a 63 percent stake in Unipetrol. A spokeswoman for Unipetrol said she was not aware of who was behind the deal.