Roku Inc. (ROKU) has one big fan betting on its success: Twenty-First Century Fox Inc. (FOXA) .

The more than 16.7 million active users streaming content on their Roku devices can breathe easy.

The media giant plans to keep its equity stake in Roku after the ink dries on Walt Disney Co.'s (DIS)  acquisition of most of Fox's properties in a deal announced Thursday, Dec. 14.

In its biggest-ever acquisition, Disney will fork over $52.4 billion for Fox's Twentieth Century Fox film and television studios and its related assets, several cable networks and international networks and stakes in National Geographic Partners, Hulu LLC, the UK satellite group Sky and more.

But Fox is holding on tight to its stake in Roku, the streaming service that's shares have doubled in price since their Sept. 29 IPO. Fox's decision to keep the stake in Roku marks a bullish endorsement from one of the most storied media empires on the market.

While Fox is shedding its holding in rival streaming service Hulu, for example, it's keeping Roku in the family. Fox, then, appears to be betting Roku's stock price will rocket even higher.

Roku shares jumped 6.23% to $48.29 Thursday. They popped another 4% Friday. For context, Roku's IPO price was set at $14 per share, but in its first day of trading jumped to about $23 by the close.

Fox first invested in Roku in 2013, nearly four years ahead of Roku's IPO when Fox was still a part of News Corp. At the time of the May 2013 investment, Roku announced it had raised $60 million in a funding round that included Fox, but didn't make it clear just how much Fox had put in.

Fox and BSkyB, which Fox had at that time owned a 39% stake in, also invested in Roku in October 2014. BSkyB said it added $700,000 to its Roku investment to bring the total invested to $12.2 million. Fox again kept mum on the total dollar amount it added.

Before the Roku IPO, Fox owned about a 7% stake in the company. It's worth noting that percentage has likely shrunk, as Roku has offered two classes of shares to investors. The 7% stake has since been diluted given the nature of Roku's listing, but as an early investor, Fox retains a greater deal of voting power than those who bought into Roku at its IPO.

Fox appears to remain a darling of Roku's investor list, even if it isn't the top equity holder. Roku CEO Anthony Wood told TheStreet in November that even as institutional television players underestimate the shift of ad dollars to streaming services, "Fox's leaders...understand."

"The TV part is the fastest-growing part of the business," Wood said. That could be reason for Fox to hold onto Roku: its properties post-Disney deal will be Fox Business Network, Fox News, Fox Sports and a group of television stations.

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