Like most jam bands, Gov't Mule relies to a huge extent on the chemistry between members.

Like most jam bands, Gov't Mule relies to a huge extent on the chemistry between members.

Founded by guitarist Warren Haynes, bassist Allen Woody and drummer Matt Abts in 1994 as a power trio, the band has changed with its membership. It's a hard-charging outfit with a focus on classic rock, soul and blues.

"It's not one of those groups that can make a change overnight," Haynes says.

When Jorgen Carlsson joined the band as its permanent bassist recently, it was only after six months of rehearsing, jamming and hanging out with the other guys. Carlsson replaced Andy Hess, who had joined in 2003.

When Woody died in 2000, it left a gaping hole in the popular band, since Woody was a distinctive, one-of-a-kind player. Haynes says that made it daunting for Hess, who brought not only different chops but a different personality to the tight-knit band.

"Jorgen's style is somewhere between Woody's and Andy's," Haynes says. "He brings some of Woody's aggressive spirit back, although he is very much his own man."

Carlsson is a native of Sweden. He arrived in the United States in 1991 and has been both a studio owner and session musician.

Gov't Mule began as a side project of the Allman Brothers Band. Their debut album, "Gov't Mule," was released the next year. The band is a favorite at music festivals such as Bonnaroo, Wakarusa, Mountain Jam, Vegoose, All Good and others across North America.

When The Dead performed in May in Mountain View, Calif., for the first time in several years, Haynes played guitar with Bob Weir, Phil Lesh and friends on the two dates, shuttling up the peninsula to San Francisco and back to play with the Allman Brothers when he wasn't on stage with The Dead, prompting the Chronicle to call him the hardest-working man in show business.

The band has released eight studio albums and several live collections while amassing a tally of more than 1,300 concerts to date. It has sold more than a million albums, more than 1.5 million dowloads (at mule.net) and received a Grammy nomination.

Keyboardist Danny Louis joined the band in 2003. But it's bassist Carlsson whose chops connect the bass lines of past and future.

"Having worked with Jorgen over the last several months," Haynes says, "we are very excited to explore some new directions while at the same time going back a little closer to our roots."