Thrifty Benjamin Franklin would have been shocked. His Colonial-era portrait on the $100 bill is causing stampede.

At the opposite end of the American currency scale, puzzled federal officials are wrestling with a different problem -- a 65 percent increase in demand for pennies, apparently caused by hoarders awaiting another upturn in world copper prices.

The American currency system is undergoing some basic changes that are causing federal managers to rethink traditional methods of distributing money to the public.

One of the most surprising new trends is the mushrooming popularity of the $100 bill, know as a "C note" to generations of gamblers.

Although many citizens rarely see one, the $100 bill accounts for almost 40 percent of the value of U.S. paper currency in circulation, recently surpassing the $20.

"The popularity of the $100 bill is incredible and it's been growing like mad," said Mike Merson, who tracks currency trends for the Treasury Department. "One hundred-dollar bills are everywhere."

The public, according to Merson's figures, held $42.6 billion worth of $100 bills as of March 31. The total pool of circulating paper currency was $111.2 billion. So 38.3 percent of the worth of all paper money was in $100 bills.

The $20 bill now represents $36.9 billion, or 33.2 percent, of all folding money. One-dollar bills are the most numerous, but account for only $3 billion, or 2.7 percent, of the value of all paper cash.

Merson and other federal officials cite criminal activities like drug traffic, prostitution and tax evasion, in addition to inflation, as major factors.

"Drug pushers, prostitutes, gamblers and others on the shady side of the street move scads of cash every day," said a Justice Department official who asked not to identified.

"When you realize that $10,000 worth of $100 bills makes a stack less than half an inch thick, it's easy to understand why they are so popular among criminal elements," said Merson.

The so-called "underground economy" also includes otherwise upstanding businessmen who prefer to avoid the prying eyes of the Internal Revenue Service.

The IRS has estimated that in 1976 individuals failed to report between $100 billion and $135 billion of income -- both legal and illegal. The government therefore was cheated out of up to $26 billion in taxes.

Inflation, which has increased wages as well as prices, has prompted some people to accept bigger bills -- like $100 -- when they cash their paychecks.

It was Franklin who remarked a couple of hundred years ago that he knew of only "three faithful friends -- an old wife, an old dog and ready money." To a growing number of people, the third faithful friend now has Ben's picture on it.

At the other end of menetary spectrum sits the lowly penny, which is disappearing from cash registers across the nation as demand for it soars.

To be sure, many pennies wind up tossed into jars to lighten the load in pockets and wallets. But officials say the coins are being hoarded by speculators.

Earlier this year, the price of copper peaked at $1.48 per pound. It has since plummeted to 90 cents Mint officials say the break-even copper price is about $1.50, and it would have to climb to about $2 a pound to make it worthwhile for speculators.

Despite the steep price dip, the Federal Reserve System, which supplies coin and currency to commercial banks, has noticed no fall in demand.

In fact, the U.S. Mint says requests for pennies are up 65 percent this year, compared with a normal yearly increase of around 8 percent.

The Mint is shipping between 50 million and 55 million pennies every working day, a jump of 20 percent over last year, according to Mint spokesman Frank DeLeo.

"There's something out there -- other than the need for coins as a medium of exchange -- that is causing this heavy demand," said DeLeo.

"Despite the copper price decline, people are still hoarding pennies," a Fed official said. "Normally, the banks at all levels have pennies coming back to them . . . but they're not getting these pennies back."

"One man," the official said, "wanted to buy a truckload of pennies -- literally -- from his local bank, and was miffed when they turned him down."