Elon Musk has a bone to pick with Apple.

In a series of tweets late Monday night, Musk said xAI would take legal action against Apple for what he says is its bias toward OpenAI on the App Store.

"Apple is behaving in a manner that makes it impossible for any AI company besides OpenAI to reach #1 in the App Store, which is an unequivocal antitrust violation," Musk wrote in an X post. "xAI will take immediate legal action."

In a post earlier on Monday, he asked why the App Store hadn't placed X higher in its rankings. Musk said X deserved the spot because it is "the #1 news app in the world."

"Are you playing politics? What gives? Inquiring minds want to know," Musk wrote.

xAI didn't respond to a request for comment from Business Insider.

Apple's spokesperson said in an email to Business Insider on Tuesday that the App Store is "designed to be fair and free of bias."

"We feature thousands of apps through charts, algorithmic recommendations, and curated lists selected by experts using objective criteria," the spokesperson said.

At press time, OpenAI's ChatGPT is ranked first on the App Store. Grok is ranked fifth.

"Unfortunately, what choice do we have? Apple didn't just put their thumb on the scale, they put their whole body!" Musk wrote on X on Monday.

OpenAI CEO Sam Altman responded to Musk's threats on Monday.

"This is a remarkable claim given what I have heard alleged that Elon does to manipulate X to benefit himself and his own companies and harm his competitors and people he doesn't like," Altman wrote on X.

Altman said the ChatGPT maker would remain "focused on making great products."

But Musk was not done.

"Apple behaves like it's owned by OpenAI — why?" Musk wrote on X on Tuesday morning. He was responding to a post containing several screenshots of banners from the App Store featuring ChatGPT.

Musk and Altman cofounded OpenAI in 2015. Musk left its board in 2018. Since then, Musk has become a vocal critic of Altman's leadership of OpenAI.