The New York-based company said it is on track to reach 1 million subscribers by the end of the year.
Early this month, Sirius, which launched its satellite service in July 2002, announced a $500 million, five-year deal with Stern. The deal means that starting in 2006, after his contract with Viacom Inc.'s Infinity Broadcasting expires, Stern will leave conventional broadcasting for the relatively new medium of satellite radio.
The move from free over-the-air broadcasts to the Sirus service, for which users pay a monthly fee, will put Stern's raunchy talk show beyond the reach of government regulators.
Content on satellite radio isn't regulated, though operators do need to obtain licenses from the FCC.
Stern's show is heard by more than 10 million listeners around the United States.
Sirius has been battling XM Satellite Radio Holdings Inc. to build satellite radio into a new business, but has been the underdog. XM says it has more than 2.5 million subscribers.
Sirius said it passed the 700,000 mark on Monday, signing up more than 69,000 new subscribers in September.
In morning trading, shares of Sirius were at $3.85, up 3 cents on the Nasdaq Stock Market.