The Beatles: Rock Band, Viacom's over-hyped and costly video game based on the music of the biggest-selling rock band of all time, has not been a big seller.

According to NPD Group the game sold only sold 595,000 units in its first 25 days on sales in the United States last month. While that is bigger than the first-month sales of either 2007's Rock Band or 2008's Rock Band 2, but far below the best launches in the music video game genre.

Analysts thought that the game would do a lot better and even after the solid but disappointing start sales of the game are slipping. The month's top- selling game was Halo 3: ODST, the latest entry in Microsoft's best-selling series, which sold more than 1.5 million units.

The Beatles: Rock Band was the No. 3 release in unit sales and No. 2 in revenue for September. The game generated $60 million in sales.