New technologies enable entrants to create business models that threaten incumbents in a range of industries. This paper extends the framework of newly vulnerable markets to explore the dynamics of competition between entrants and an incumbent through a study of the New York Stock Exchange. For over 200 years, the NYSE has operated a physical market for trading securities. Beginning in the 1960s, it employed information technology to process increasing trading volumes and to disseminate data on stock prices and volumes. The Exchange invested heavily in IT for its trading floor, and defended it against electronic markets enabled by new technologies. In 2005 various pressures forced the NYSE into a merger with Archipelago, a leading electronic exchange. The NYSE's latest market system, Hybrid, launched in 2006, is the first at the NYSE that enables investors to bypass the trading floor completely. Despite the new technology, its share of trading volumes fell. This paper documents four eras of IT management at the exchange, presenting a detailed 27-year history. The evidence shows that while IT investments helped the Exchange defend floor trading and its market share for a number of years, it finally had to adopt the technology of all-electronic exchanges. (C) 2009 Elsevier B.V. All rights reserved.