CMC Magnetics (TPE:3050) sold 1.1 million shares in Nanya Technology (TPE:2408) for about NT$319 million, according to a Wednesday Taiwan Exchange filing.
Shares gained over 5% in Thursday's midday trade.
The shares were sold between May 18 and May 20, at an average price of NT$295.40 per share.
The company attributed the stock sale to its routine asset management and portfolio rebalancing guidelines.
Following the transaction, the company holds a 0.11% stake in Nanya Technology, it said.