Investors have richly rewarded a pair of companies that are bringing electronic trading to the bond market, which has long been dominated by Wall Street banks making deals by phone, email and instant message.

Shares of MarketAxess Holdings, which runs a major electronic platform for trading corporate bonds, rose 2.3% on Wednesday, a day after the New York-based company reported strong June trading volumes. Its stock has climbed 60% since the beginning of the year, while the S&P 500 has gained just under 20%.