Note: This story first ran with an incorrect figure for National Bedding’s sales. It has been corrected.

The private-equity arms of L.A.-based Ares Management LLC and the Ontario Teachers’ Pension Plan agreed on Tuesday to purchase National Bedding Co., maker of Serta brand mattresses.

Financial details of the transaction were not disclosed. A source said the purchase price was more than $500 million but less than $1 billion. National Bedding, based in Hoffman Estates, Ill., has annual sales of more than $600 million.

The purchase was made by Ares Corporate Opportunities Fund and Teachers’ Private Capital, part of the Canadian pension fund Ontario Teachers. National Bedding founders Burt Kaplan and Richard Yulman will remain investors and directors of National Bedding.

“We look forward to the opportunity to increase our ownership stake as well as to continue to support the company’s growth in the $5 billion bedding industry,” said Bennett Rosenthal, a partner in Ares Management, in a statement.

National Bedding holds 27 of 34 exclusive Serta manufacturing licenses and accounts for nearly 80 percent of all sales under the Serta name in the U.S. and Canada. Serta is the third largest U.S. manufacturer of mattresses.

Ares Management has had a minority stake in National Bedding for two years and is an investment firm with more $7 billion of committed capital under management. Teachers’ Private Capital invests on behalf of 255,000 active and retired teachers in Ontario, Canada.