The story of Kevin Plank and his company, Under Armour Performance Apparel, has so far been a feel-good tale: The former University of Maryland fullback comes up with a new kind of athletic clothing that sweeps the nation and lands him contracts with everyone from the National Hockey League to the United States Ski Team and makes his product a must-have among the Little League set.

But now comes the part in the story where the battle really begins. The 30-year-old Plank has awakened sleeping giants. Nike Inc. and Reebok International Ltd. are setting their sights on his niche by introducing their own lines of similar apparel.

Sick of being drenched in sweat while wearing cotton T-shirts during practice and games, Plank was determined to make shorts and undershirts out of synthetic fibers that would wick moisture away from athletes as they perspire.

Under Armour began in his late grandmother's house in Georgetown seven years ago, shortly after he graduated with a business degree. Plank was the sole employee.

Now, his company is in its third Baltimore location, a sprawling 33,000-square-foot office in Tide Point with sweeping views of the harbor and Fells Point, and Plank has about 175 people working under him. Under Armour contracts out its manufacturing, he said.

Sales have grown from $13,000 in 1996 to more than $55 million this year, and Plank expects to double that next year.

Still, it's nothing near the $10 billion and $3 billion raked in respectively by Nike and Reebok last year, who both now have their own lines of moisture-reducing undergarments.

So far, Plank doesn't seem too worried about the competition.

"We can't control what Nike and Reebok do," he said. "We can only control what we can control."

Industry experts say he has reason to be optimistic about his chances for success even against these Goliaths. His brand has made huge inroads with both professional and amateur athletes and has bored itself into their minds as the product when it comes to technical apparel, the industry term for high-grade sports garments.

"You have to remember, fanatics are also loyalists," said Marshal Cohen, co-president of the apparel division at NPD Group, a marketing research company. "When you've endorsed a brand in your mind, you will go to the ends of the Earth to buy it until something triggers you to switch. ... I think [Under Armour is] one of the great examples of future success stories in the sports apparel arena."

Between the first of the year and Nov. 24, Under Armour had 93 percent of the market in compression shirts and 23 percent in compression shorts, according to the research firm SportScan Info. Nike and Reebok had less than 5 percent of the sales in either category.

The buyer for men's and children's branded active wear at Hibbett Sporting Goods, an Alabama retailer with 370 stores in the Southeast, said the chain carries Nike, Reebok and Under Armour apparel but that the two mammoths have nothing on the Baltimore firm.

"It is by far and away the one and only - it's unbelievable," said buyer Susan Wilson. "I can truthfully say all peewee football players have got a piece of Under Armour; it is a must have and even at $30 a pop, mom is buying it for them. It's become as important to kids - from 8 to 10 years old or 16 to 17 - as their helmets."

John Horan, publisher of the newsletter Sporting Goods Intelligence, called Under Armour a "hot little company" that has built up a lot of credibility with athletes. But he noted that Reebok in particular could present a huge problem for Under Armour.

Reebok is the official supplier of apparel for the National Football League - meaning players are supposed to be seen on camera wearing only Reebok's brand - and this year Reebok launched an "NFL Equipment" apparel line that is made from "PlayDry" material similar to that of Under Armour's products.

"This is really our first season and we're noticing a lot of people getting familiar with it, a lot more people are asking for it now," said Chris Gallo, product manager for the Reebok line.

"I think Under Armour has done a great job. They're a small company with a great brand. But we're building this product line, we're going to bring it out to the retail end and I think we're going to be successful."

Reebok would not disclose sales figures for its NFL line.

Plank said he considers the cotton industry, not Reebok or Nike, his main competition - along with his own success and the rapid changes it has brought to Under Armour.

"Our biggest concern is probably ourselves," he said. "There's a lot of management that needs to take place, and we're a different company every three to six months."

The brand, which never goes on sale, is sold in about 3,000 stores nationwide, including Dick's Sporting Goods, the Sports Authority and Galyan's.

Plank said he prefers to expand the Under Armour line within existing stores rather than have his products sold at deep discounts in lower-end retailers such as J.C. Penney Co. Inc. or Kohl's Corp.

Next month, for example, Under Armour will introduce several new products, including a women's line, men's boxers, boxer briefs and an expanded youth line.

Plank said he's focused on expanding the company and is not interested in going public - "we don't want to have to answer to the pundits on Wall Street" - or in being acquired.

"I'm a dreamer, and dreamers are tough to negotiate with because they have a high opinion of what they've created," he said.

"At this company, we have a blast and that's something pretty special. ... If someone offered me $5 billion, then sure, but I'm having too much fun doing what I'm doing."