Fidelity is rolling out fractional trading in stocks and exchange-traded funds.

The firm’s retail clients will be able to trade, in real time, as little as 0.001 of a share through the firm’s mobile apps, available for iPhones and Android phones, according to a press release from Fidelity.

The rollout will continue over the next several weeks but fractional trading is already available in several “eligible” accounts, including IRAs, brokerage accounts, HSAs and self-directed brokerage accounts in workplace retirement plans, the company says.

Fidelity touts fractional trading, also known as dollar-based investing, as a way for investors with even limited funds to better diversify their portfolios as well as take advantage of dollar-cost averaging.

Other firms have also looked to fractional trading to lure mass affluent clients.

In October, Charles Schwab’s CEO told the Wall Street Journal that the company plans to offer the option, although he didn’t say when. In November, JPMorgan announced a pilot program that allows fractional trading in ETFs for certain accounts with less than $5,000. And startups M1 Finance and Social Finance already offer fractional trading.

The real-time aspect of its fractional trading offering is what Fidelity says sets it apart.

All the trades will be executed in real time and during market hours, “meaning customers will always know the share price, unlike some firms that execute fractional trades at the end of a trading day or wait for multiple orders to add up to full shares,” Fidelity says in the press release.

The move also comes as more and more firms compete for mass affluent clients with the lure of zero commissions. Fidelity, which now has more than 23 million retail brokerage accounts, according to the company, eliminated commissions for online trades in stocks, ETFs and options in October, following similar moves from Schwab, TD Ameritrade and E*Trade.

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