SAN FRANCISCO — While a number of large financial institutions have discussed trading Bitcoin, one firm has already begun doing it. Very quietly.

The financial firm, Susquehanna International Group in Bala Cynwyd, Pa., just outside Philadelphia, is one of the largest players in trading traditional investments like stocks, options and exchange traded funds, or E.T.F.s. Over the last two years, the privately owned company has also built up a trading desk of around a dozen people that buys and sells millions of dollars’ worth of Bitcoin and other virtual or cryptocurrencies in private deals.

Now the firm is opening trading to a small group of its 500 clients, with plans to expand.

The move is the latest sign that the virtual currency markets, which were once relegated to the fringes of the financial world, are being embraced by big, mainstream investors.

The parent company of the New York Stock Exchange, the Intercontinental Exchange, has been in talks about opening a subsidiary for cryptocurrency trading, and Goldman Sachs is on the verge of opening its own trading operation.