Blackboard Inc., a District company that sells online education software, has agreed to buy its biggest rival, WebCT Inc., in a $180 million deal that strengthens eight-year-old Blackboard's position in a growing market.

If the cash deal is approved, Blackboard will absorb Lynnfield, Mass.-based WebCT and its 1,480 customers.

At the core of both companies are software products that help teachers put coursework online and conduct discussions and tutorials through the Internet. Together, the two firms control about 80 percent of the U.S. market for sales of online learning systems.

"You put the No. 1 and No. 2 together in a space without a whole lot of credible competition, and they're in good shape," said Bradley L. Mook, an analyst with Boenning & Scattergood, an investment banking firm.

Eduventures, a market research firm in Boston, estimates the market for online learning software at $220.7 million this year. Blackboard said it expects its revenue to be $134 million to $135 million this year, with profit of $24 million to $24.6 million.

While privately held WebCT, which has landed $125 million in venture funding since it was founded in 1996, has remained competitive with Blackboard in sales of course-management software -- it has about 35 percent of the market, according to Market Data Retrieval -- analysts say Blackboard's June 2004 initial public offering and its expansion into other products have given the District company significant advantages.

Blackboard, founded in 1997, also provides software to manage university portals and student transactions on campus. Michael L. Chasen, the firm's chief executive, said Blackboard will offer those products to WebCT's customers as well as both companies' versions of the course management software.

Analysts say the biggest benefit to Blackboard from the purchase is in protecting its stake in the international market. While the U.S. higher education market for online learning software is nearly saturated -- about 88 percent of U.S. colleges have a system in place, according to Eduventures -- adoption of the technology has been slower overseas, where both companies have about 500 clients.

"It's a positive for [Blackboard] because it allows them to accelerate the pace of growth overseas, but it's also a defensive move, because it prevents someone else from coming in, buying WebCT and presenting a bigger challenge to them overseas," said Trace A. Urdan, an analyst with Robert W. Baird & Co.

Chasen said the company will maintain WebCT's offices in Lynnfield and Canada and will retain most of its 270 employees.

Shares of Blackboard closed at $22.92, down 97 cents.