When Sibyl White started working as assistant to the president at Denver's Mammoth Micro Productions in April, the office environment took some getting used to.

Previous employers had required her to wear suits and dresses. In her new job, people were walking around the office in jeans and T-shirts. Some in their stocking feet."It has been hard for me," says White, who also has worked for Mary Kay Cosmetics and Disney. "I don't think I could wear shorts to work. Jeans, yes, and a nice shirt."

When clients come to call, White and her bosses dress up, but it's not a button-down kind of place.

Mammoth makes CD-ROMs and is typical of the computer industry, which years ago dumped dress codes in favor of a more worker-friendly environment. Slowly but surely, the rest of the business world is following suit.

A 1992 study commissioned by Levi-Strauss found that 67 percent of American companies permit casual dress in the office on an everyday or seasonal basis, or on designated "casual days."

"This phenomenon is affecting any number of companies and every type of business," says Debbie Dispense, marketing and sales director for Visual Dimensions, an image consulting firm.

Dispense cites several reasons for the growth of casual corporate style. Corporate management is becoming more decentralized, with many ideas coming from the worker ranks. In addition, as baby boomers hit upper management, they bring to the executive suite a different style, as well as their expanding waistlines and a preference for casual clothes.

There's also an economic advantage. When an employee can wear the same shirts, pants and skirts for work and leisure hours, the need for a separate career wardrobe is eliminated (as well as the expense of dry cleaning and shirt laundering).

Employees say their morale improves when they dress casually, they think it improves productivity and they perceive it as a job benefit, according to a recent survey commissioned by Levi Strauss. But the whole phenomenon can become confusing. Everyone seems to have their own view of what's casual. Khakis and polo shirts may be as relaxed as one guy gets, while another insists on jeans and a T-shirt. What works for one business or industry may be wrong for another.

Dispense, the image consultant, cautions that casual day shouldn't be viewed as an opportunity to express the side of your personality that you normally unleash only in the company of close friends or on the dance floor. The employee who dresses in somber suits four days of the week and then lets her hair down - literally as well as figuratively - on the fifth will send a mixed message to her superiors.

Some guidelines to consider include: Is the outfit appropriate for you and your profession? Is it neat, clean and well-pressed? Is there balance from head to toe?

"You have to remember that you cannot dress in a neutral way," Dispense says. "Your clothes are always saying something about you. Research shows that non-verbal communication has five times the impact of verbal communication, and that first impressions are made in 15 to 30 seconds in a one-on-one situation.

"We're not saying you ought to wear suits. We're saying you should stop and think about what nonverbal message you're giving."

Also, think about what will happen down the road, she says. What kind of image will the company president have of you when it's time to hand out promotions?

And that's not to say that you should dress like your superiors, either, because they may not be presenting the best image.

In companies without written casual dress policies, a rule of thumb is to dress in the same way as the people with whom you are doing business. If they wear conservative suits, so should you.

"What people wear will change throughout the week," says Michael Rounds, media relations manager for Cyprus Amax Minerals Co., a mining company that instituted a casual dress policy in April. "It depends on if they're here or off-site.

"It's left to the discretion of each department as to what the employees should wear," Rounds says. "Most of us on the fourth floor wear coats and ties, but those who don't have as much outside contact are considerably more relaxed."

At Amax, that doesn't mean blue jeans, but pants and an open-collared shirt, Rounds says. "We've always been conservative."

Ditto for IBM, where business casual has made big inroads in a corporation that long has epitomized the dark suit-white shirt tradition.

"I've been with IBM almost 30 years, and it was standard to wear a suit and tie when dealing with customers," says Bill Newman, state external programs manager.

That's because the executives IBM salesmen were calling on were dressed the same way. Now that the marketplace has relaxed, so has IBM. Says Newman, "Business casual is appropriate much of the time."