A company formed by a co-owner of the Apthorp has acquired nine buildings on the Upper West Side and the West Village from a family firm that owned them for more than six decades.

Northbrook Partners LLC paid more than $300 million for the properties, the company’s founder, Maurice Mann, who is also president of Mann Realty Associates, said. Papers transferring ownership of the buildings were signed last Friday, he said.

Mr. Mann formed Northbrook Partners in 2006 as a partnership between Mann Realty and several real estate investment trusts.

Mann Realty bought the Apthorp in 2006 for $426 million, or an average of $2.4 million for each apartment, and last year formed a 50-50 partnership in the building with Africa-Israel USA, which is also paying for a $95 million renovation of the historic apartments.

At least two of the nine buildings that Northbrook Partners acquired are rentals. One has about 130 units at 98 Riverside Drive and the other has about 160 units at 48-60 Seventh Ave., residents of the buildings said. The deal also includes the remaining shares of a building recently converted to condominiums at 172 W. 79th St. Details on the other buildings were not available as of press time.

The seller was AVJ Realty, a firm owned by the heirs of the Racolin family, which bought or constructed a slew of buildings in the city during the 1930s and 1940s. A spokesman for the family, Joshua Martinsons, confirmed it has sold “several buildings,” but declined to describe the details of the deal.

“We are very sorry to be longer the owners of the projects,” Mr. Martinsons said. “But we feel that it they are going into excellent hands. We had other matters to attend to in all of our lives.”

Mr. Mann said he had no plans to convert any of the buildings, but he said he “bought with an eye toward doing improvements.”

Several residents of 98 Riverside Drive said they were worried about rent increases. About 20% to 25% of the building has market-rate apartments and the remaining apartments are either rent-controlled or rent-stablized, they said. “When you have a company that has a Web site discussing how they turned a $100,000 rent roll into $800,000, you get a little worried,” a vice president of the building’s tenant association, Gary Schatzky, said. The Racolin family declined to sell the building to the current tenants, he said.

The turnover to market-rate apartments wouldn’t be a quick one, the president of the association, Karol Stonger, said.

“People leave this building feet first,” she said. She described the residents as community-oriented and mostly from middle-class backgrounds. Many of the market-rate tenants were relatives or friends of the Racolin family.

Mendel Racolin was a Russian dentist who entered into the real estate business in about 1930, according to an obituary in the New York Times on March 31, 1950. He died at 69.

The article described him as a patron of the Jewish Scientific Institute and the Jewish Encyclopedia, as well as a co-sponsor of a project to house Jewish children orphaned by the Nazis in Paris called the Vladeck Homes.

At the peak of his real estate career, he owned buildings in the Manhattan, the Bronx, and Queens, but the family sold most of the property in the decades after his death.

Northbrook Partners, together with the investment firm Black-Rock, also bought a 139-unit at 530 Park Ave. last fall, The New York Sun reported. Mr. Mann’s Web site says Northbrook Partners “manage well over $1 billion worth of high-end real estate in Manhattan.”