BACK in the 1970s, after American regulators abolished fixed commissions for brokers who helped their clients trade shares, the likes of Charles Schwab and Fidelity were the insurgents. They dispensed with the expensive frills that most rivals offered, such as research and investment advice. That, in turn, allowed them to offer share trading to the masses at bargain prices. Twenty years later, the internet spurred the growth of a new wave of discount brokers, including E*Trade and TD Ameritrade. Now for the next challenger.

Whereas full-service brokers demand a percentage of the value of the assets in their clients’ accounts (typically 1-1.75% a year), the discount firms charge around $9 a trade. That is highway robbery, however, by the standards of a new online brokerage, Robinhood, which enables clients to trade shares free of charge, via a new mobile app.

Instead of taking commissions from customers, Robinhood receives them from the trading venues to which it steers their orders, a controversial but common practice. It also earns returns from the cash clients leave in their accounts, and plans soon to offer margin trading—the buying of stock with borrowed money—for which it will charge a fee.

Whether this is enough to cover Robinhood’s costs is unclear (the firm does not disclose its financial results). But it provides an even leaner service than its rivals. It does not yet offer trading on its website, for example, catering to clients, whose average age is 28, exclusively via its app.

Robinhood is also easy to use. Setting up an account can take as little as four minutes. To confirm customers’ identity, the firm asks them to take a picture of an ID with a smartphone. That has helped it to attract almost 1m customers since it started operations a year ago. It says it continued to recruit new clients during the recent market turmoil, even as activity declined at other firms. To bring in even more custom, it has begun integrating its service with apps such as Stocktwits, a social-media platform for retail investors. Increased turnover should boost its income more than its costs. In time, it may make life difficult for the disruptors of yore.