Exelon agreed to buy Constellation Energy in a $7.9 billion all-stock deal Thursday, the latest in a string of tie-ups in the electric power industry.

The deal pairs Exelon, one of the largest wholesale power sellers in the country, with Constellation, one of the largest retail power sellers.

A combination of lower power prices and rising costs tied to tightening environmental standards has been the catalyst for several recent deals.

Exelon Corp., based in Chicago, owns the Illinois utility ComEd and the Pennsylvania utility PECO, along with a large wholesale power division anchored by the nation's largest fleet of nuclear plants.

Constellation, based in Baltimore, owns the Maryland utility Baltimore Gas and Electric. It also owns power plants in 13 states and sells power to commercial and residential customers across the country.