Bolstering its high-end, retail energy business, Constellation Energy Group Inc. said yesterday that it will buy a Virginia-based commercial and industrial energy-services company for $240 million to add about 3,000 large electricity customers to its client list.

The purchase of AES NewEnergy, a subsidiary of AES Corp. in Arlington, Va., is expected to immediately add to Constellation's earnings when the transaction is completed. The cash deal is expected to close at the end of the year.

It will be Constellation's first major purchase since Mayo A. Shattuck III took over as chief executive in November after the company canceled a plan to split into two businesses.

"It's a very good fit for our company," said Charles B. Welsh, a Constellation spokesman. "It's part of an evolving strategy to serve large and industrial customers in deregulated markets where we already have extensive wholesale electricity experience.

"Their customers use large amounts of electricity, similar to the wholesale customers we have now, such as other distribution utilities, electric cooperatives and municipalities," Welsh said.

Last month, Constellation purchased Enron Corp.'s contracts with customers in Maine, which had been in bankruptcy hearings. Constellation gained about 500 industrial and commercial customers from that purchase.

Here in its own territory, Constellation subsidiary BGE Commercial Building Systems has focused on winning energy contracts with large Central Maryland commercial and industrial customers, which will be forced to find alternative energy suppliers this month when a price freeze ends for them.

NewEnergy, which is to be renamed and become a subsidiary within Constellation, is a leading national provider of electricity, natural gas and energy services to customers in deregulated markets across the country, including the Northeast, the mid-Atlantic region, the Midwest, Texas and California.

Constellation would not disclose yesterday details of the cash financing for the transaction, which will cover AES' total current investment in the business.

NewEnergy has about 260 employees and offices in each market. Welsh said, "We don't have any plans at this time to consolidate or expand, open new offices or close offices," he said. "It would be premature to make any decisions about that at this stage."

The purchase is subject to approval by the Federal Energy Regulatory Commission, the Securities and Exchange Commission, and the U.S. Justice Department under the Hart-Scott-Rodino Anti-trust Improvement Act.

Shares of Constellation rose 6 cents yesterday on the New York Stock Exchange to close at $27.81.