On weekend mornings on fashionably casual Montana Avenue in Santa Monica, the best way to disappear in the crowd is to carry a vitamin-fortified juice smoothie in one hand and a rolled-up rubber mat under the other arm.

The mat is a giveaway that its owner practices yoga, an activity that has made a steady comeback in the '90s and is now shifting into higher gear as a business.

Stylish, airy yoga centers--including at least one franchise chain--are opening throughout Southern California. More than 70 now exist, charging as much as $1,200 a year for classes.

"The rise is steady. People are just tired of killing their knees with jogging and aerobics," says Maty Ezraty, who owns Yoga Works on Montana and recently opened a second studio on Main Street on the other side of town.

Incense and Eastern mysticism are downplayed at many of the new yoga centers; T-shirts and baseball caps emblazoned with the chains' logos are in. They are being snapped up by aging baby boomers and a younger breed of yoga enthusiasts who need to be told to turn off their cellular phones before they can start doing the plow or sitting in the lotus position.

Invented 5,000 years ago by Hindus, yoga is a discipline that involves special postures, breathing exercises and stretching. In traditional yoga, these physical exercises are linked to meditation. But most new yoga buffs enjoy it mainly for its gentle toning and calming qualities.

Actress Julia Roberts is among this new class of customer.

"Yoga is my new fascination. I doubt I'll ever become all about yoga. I don't want it to change my life. Just my butt,"

she recently told In Style magazine.

Yoga is enjoying a renaissance in the United States following its brief popularity in the 1960s. A Jane Fonda video three years ago helped relaunch it into the American mainstream, but observers agree that only in the last 18 months has yoga started to show the signs of a genuine craze, complete with franchise operators and merchandising efforts.

"It's very metropolitan and upscale, and it is going to be a strong trend," said John McCarthy, executive director of the International Health and Racket Sports Assn., a Boston-based group representing more than 3,000 fitness clubs around the world.

"The snowball has gotten so big that now it's impossible not to notice it," added Rod Stryker, co-owner of a Yoga Zone that opened six months ago in Brentwood.