Sirius Satellite Radio Inc. named former Viacom Inc. President Mel Karmazin chief executive a month after signing talk show host Howard Stern to lure listeners. Sirius shares rose as much as 19%.

Mr. Karmazin was Mr. Stern’s boss at Viacom’s Infinity radio, where the announcer had the top-rated show among young men in New York and Los Angeles. Mr. Karmazin succeeds Joseph Clayton, who will remain chairman of the board, New York-based Sirius said in a statement. Sirius signed Mr. Stern on October 6 to a five-year, $500 million contract.

Sirius and XM Satellite Radio Holdings Incorporated are trying to lure subscribers and reverse losses, spending millions of dollars to acquire new programming. By hiring Mr. Karmazin, the company gains an executive with almost four decades of experience in the radio industry who built Infinity into a national broadcaster.

“Sirius shareholders, customers, and employees should be very happy,” said Michael Holland, chairman of New York-based Holland & Co., which manages $500 million and doesn’t own Sirius shares. “He produces for all three constituencies. It’s a big deal.”

Mr. Karmazin didn’t immediately return telephone and e-mail messages seeking comment.

“This is a perfect opportunity for me because I want to lead a growth company that can reshape the landscape of the radio business,” Mr. Karmazin said in a statement.

Sirius shares rose 75 cents to $5.70 at 5:21 p.m. after the close of trading on the Nasdaq Stock Market. The shares closed yesterday at $4.72.

On June 1,Mr.Karmazin quit Viacom after four years of tension with Viacom’s chairman and chief executive officer, Sumner Redstone, over who made day-to-day decisions at America’s third-largest broadcasting, entertainment, and advertising company.

Mr. Karmazin was frustrated by the performance of some business units, including the radio stations, and the decline in the company’s stock, Mr. Redstone said on a conference call at the time.

Mr. Karmazin, who began his career in radio advertising sales in 1967, led Infinity before the radio broadcaster was bought by Westinghouse Electric Corp., which later took the CBS name. Viacom acquired CBS for $43.4 billion.

Mr. Stern said when Mr. Karmazin quit he was “shocked” by the departure and described it as a “coup d’etat.”

In the second quarter, Sirius had a loss of $136.8 million on sales of $13.2 million. XM Satellite Radio said on August 5 its second-quarter loss widened to $166.1 million as costs rose.

Sirius had 700,000 subscribers as of October and reiterated its goal of reaching 1 million users by year-end. It has said it needs 2 million to break even. The company agreed to pay $220 million to broadcast National Football League games.

The trade publication Advertising Age reported Mr. Karmazin’s hiring earlier yesterday.