With the FAANG stocks—Facebook Inc. (FB), Amazon.com Inc. (AMZN), Apple Inc. (AAPL), Netflix Inc. (NFLX) and Alphabet Inc. (GOOG)—surging all year long, bears are upping their bets that the stocks will eventually come down.

That is particularly true since the middle of August, with S3 Partners, a financial analytics firm, finding short interest for the FAANG stocks is up $1.93 billion or 7.6% since the middle of last month. “As the market continues to hit high’s almost daily, these in favor companies could see a higher demand to add to shorts,” wrote S3 Partners analyst Brett Weiss in a research report Thursday. “Should investors want to increase these bets, there is plenty of stock to borrow. “ (See also: FAANG Stocks Reignite Mutual Funds.)

Since the start of 2017, the FAANG stocks have been having a good year with all of them lodging double-digit gains. Take Facebook: Shares have been climbing thanks in large part to the company’s continued ability to grow its user base and get more advertising dollars from marketers in the U.S. and abroad. This past summer it reached 2 billion monthly active users and is now on the hunt for the next billion.

When Up Equals Down

Meanwhile Apple’s shares had been surging, leading up to its iPhone 8 and iPhone X event on Sept. 12, but has since retreated a bit over concerns about demand for its newest gadgets. E-commerce giant Amazon has been having a blowout year, recently completing its multi-billion-dollar deal to acquire Whole Foods Market and just this week announcing a new line of Echo smart speakers just in time for the holidays. Google, the main driver of Alphabet’s stock, has also had a strong year so far, remaining in the dominant position in online advertising, although it has taken some hits, namely the record $2.7 billion fine handed down by the European Union this summer. Netflix is also facing some pressure after the Walt Disney Co. (DIS) recently announced it was pulling its movies and content from the streaming service as it gears up to launch its own on-demand platform. Despite the bumps along the way, all of the FAANG stocks have been outperformers, which is drawing in the bearish bets. (See also: Why the FANG Stocks Will Dominate Long Term.)

While investors betting the shares will go down are negative on the entire group, S3 did say short interest is particularly pronounced with Facebook, Amazon and Netflix. Short interest since the middle of August has increased $494 million for Facebook, $502 million for Amazon and $910 million for Netflix. “All of these companies continue to trade at general collateral levels, the cheapest fee or easiest to borrow stocks,” wrote the analyst at S3. “With the S&P 500 Index just off its all-time high of 2,508.24 and bearish investors adding to an already elevated level of short interest, the outlook for the FAANG stocks is troubling.”

Want to learn how to invest?

Get a free 10 week email series that will teach you how to start investing.

Delivered twice a week, straight to your inbox.