<div id="readability-page-1">Billionaire investor Bill Ackman made a name for himself in the 21st century through activist short-selling. But these days, Ackman's fund, Pershing Square Capital Management, tends to own about a dozen stocks at any given time, usually on the long side. Pershing has never owned Tesla (TSLA 4.77%), the electric car and robotaxi company run by CEO Elon Musk. Ackman is not actively shorting the stock or purchasing put options on the name, either. But Pershing launched a new position earlier this year, which is now the fund's largest holding. And it's essentially a bet against Tesla -- at least for now. <h2>The robotaxi revolution has begun</h2> Robotaxis with full-self-driving (FSD) capabilities are beginning to pop up in some of the largest cities in the country. After years of experts predicting that FSD would one day be ubiquitous, it feels like the industry is now at a tipping point. <div><img src="https://imageproxy.pbkrs.com/https://g.foolcdn.com/image//query-b3A9cmVzaXplJnVybD1odHRwczovL2cuZm9vbGNkbi5jb20vZWRpdG9yaWFsL2ltYWdlcy84MjM0MjMvYm9hcmRyb29tLTIuanBnJnc9NzAw?x-oss-process=image/auto-orient,1/interlace,1/resize,w_1440,h_1440/quality,q_95/format,jpg" width="700" height="467" original-src="https://imageproxy.pbkrs.com/https://g.foolcdn.com/image//query-b3A9cmVzaXplJnVybD1odHRwczovL2cuZm9vbGNkbi5jb20vZWRpdG9yaWFsL2ltYWdlcy84MjM0MjMvYm9hcmRyb29tLTIuanBnJnc9NzAw"/>Image source: Getty Images. </div> Tesla plans to play a big role in the FSD revolution. Musk has been talking about it for years, and Tesla recently launched its first batch of autonomous robotaxis in Austin, Texas, in what essentially looks like a soft launch. It's still early. Tesla's first robotaxis are geo-fenced, meaning they are only operating in parts of Austin, and the vehicles are also being overseen by humans remotely. But investors think this could be the beginning of a new robotaxi fleet that could potentially cut into the ride-hailing business. Speaking of ride-hailing, Pershing earlier this year acquired a large stake in the ride-hailing giant Uber (UBER -0.11%), which was Pershing's largest equity holding at the end of the first quarter, consuming 19% of the portfolio. In a post on X in February, Ackman praised the work of Uber's CEO Dara Khosrowshahi, who &#34;has done a superb job in transforming the company into a highly profitable and cash-generative growth machine.&#34; Ackman called Uber one of the highest-quality businesses he's seen in the market that could be purchased cheap compared to what it's really worth. While the billionaire investor may not have said it, a big bet on Uber is currently a big bet against Tesla. That's because both companies are banking on robotaxis to carry their future businesses, albeit in different ways. Tesla is building FSD electric vehicles and planning to launch its own robotaxi fleet. In a research note in late May, Wedbush analyst Scott Devitt said, &#34;We continue to believe Tesla Robotaxi serves as a long-term threat to Uber's business model... The initial launch may ramp slowly initially but 'gradually then suddenly' is a quote investors may be prudent to contemplate.&#34; Not only could a robotaxi fleet without the cost of human labor challenge Uber, but Uber itself is betting on robotaxis with its plan to build a robust network of partners. The company has already forged partnerships with Waymo, Pony.ai, and WeRide, among others. The plan is to launch autonomous vehicles from partners through its network. In a prior presentation, Uber believes the autonomous opportunity could be worth more than $1 trillion in the U.S. alone. Furthermore, management believes the company is uniquely positioned to take advantage of the autonomous wave because companies building these vehicles will need a partner. Uber's massive platform and scale, operational capabilities, and experience on the regulatory side make it an obvious choice. <h2>Is Tesla a potential partner or future competitor?</h2> In its presentation, which is from February of this year, Uber lists Tesla as a potential partner. Earlier this year, some media outlets reported that Tesla was contemplating an acquisition of Uber. However, Musk has shut down those rumors, saying there is &#34;no need&#34; to buy Uber because Tesla has plenty of vehicles it can use for its fleet. Additionally, Musk envisions a scenario where Tesla owners can lend their vehicles to the fleet and earn passive income. This more or less positions the two companies as competitors in the robotaxi space -- at least for the time being. Could Tesla one day become a partner of Uber? Possibly, especially if Tesla's fleet struggles to scale. Perhaps there is room for more than one to thrive in the space, although, in the current ride-hailing sector, Uber and Lyft essentially control the entire market. In several instances this year, good news for Tesla's robotaxi fleet has sent shares of Uber down. It's hard to know how things will shake out in such early days, but with Tesla trading at close to 170 times forward earnings, investors are clearly baking in tremendous success in the robotaxi business. Uber, trading at 25 times forward earnings, is clearly not baking in as much success in its autonomous ambitions. That's something for investors to keep in mind as they evaluate both stocks. </div>

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Billionaire investor Bill Ackman is currently betting against Tesla through his fund, Pershing Square Capital Management, which has launched a new position that is now its largest holding. While not actively shorting Tesla, Ackman's investment in Uber positions him against Tesla's robotaxi ambitions. Both companies are competing in the robotaxi space, with Tesla launching its first autonomous vehicles in Austin. Analysts suggest Tesla's robotaxi fleet could threaten Uber's business model, as both companies aim to capitalize on the autonomous vehicle market.

- Bill Ackman's Pershing Square Capital Management has taken a significant position in Uber, which is now its largest holding, while not investing in Tesla, despite both companies' focus on robotaxis.  
- Tesla has launched its first autonomous robotaxis in Austin, Texas, marking a potential shift in the ride-hailing industry, while Uber is also pursuing partnerships to develop its own autonomous vehicle network.  
- The competition between Tesla and Uber in the robotaxi market raises questions about their future roles, with Tesla's high valuation reflecting investor optimism compared to Uber's more conservative earnings outlook.

Billionaire Investor Bill Ackman Is Betting Against Tesla -- At Least for Now