Elon Musk has accused Microsoft co-founder Bill Gates of shorting Tesla.

In a tweet on Friday, the Tesla CEO admitted that he asked Gates if he was short-selling shares of the electric carmaker. When investors short a stock, they are betting that the price of the asset will fall.

“I heard from multiple people at TED that Gates still had half billion short against Tesla, which is why I asked him, so it’s not exactly top secret,” Musk said in the tweet.

He was responding to a Twitter user’s question on whether a screengrab of a supposed text conversation between the two billionaires was real.

The Tesla chief’s response was: “Yeah, but I didn’t leak it to NYT. They must have got it through friends of friends.”

In the text exchange, which couldn’t be independently verified by CNBC, Musk asked Gates: “Do you still have a half billion dollar short position against Tesla?”

To which Gates replied: “Sorry to say I haven’t closed it out. I would like to discuss philanthropy possibilities.”

Musk shot back: “Sorry, I cannot take your philanthropy on climate change seriously when you have a massive short position against Tesla, the company doing the most to solve climate change.”

A spokesperson for Bill Gates was not immediately available for comment when contacted by CNBC.

Gates told NYT opinion writer Kara Swisher last year: “It’s important to say that what Elon did with Tesla is one of the greatest contributions to climate change anyone’s ever made. And you know, underestimating Elon is not a good idea.”

But he went on to add that what Tesla was doing was “easy stuff, like passenger cars.” Gates emphasized the need to make a greater impact on climate change by tackling other industries.

“We’re basically not doing enough on the hard stuff: steel, cement, meat,” he said at that time. “And sadly, the things people think about — the electricity, passenger cars — are a third of the problem. So we have to work on the two-thirds.”

“If all you pay attention to is those short-term metrics, not the green premiums across the board, then you miss out on what is the longest lead time, which is the hard stuff.”

It’s not the first time the two men have had a public disagreement.

When Gates revealed in 2020 that he bought himself an all-electric Porsche Taycan, a Twitter user asked: “I wonder why Bill Gates decided to go with the Taycan instead of a Tesla.”

Musk responded in a tweet: “My conversations with Gates have been underwhelming tbh [to be honest].”

Musk has also previously speculated on the possibility of Gates shorting his company’s stock. Asked about those comments and whether he was short Tesla, Gates told CNBC last year: “I don’t talk about my investments but I think he should be very proud of what he’s done.”

In a Bloomberg interview also in February 2021, Gates said that he wished he had “been more on the long side” of Tesla when asked about Musk’s claims.

— CNBC’s Todd Haselton and Ryan Browne contributed to this report