In the annals of history, the name of Elias Bermudez may never rank with King Canute, who is supposed to have commanded the tides to rise no further, but in his small way, Bermudez is following the king's footsteps.

Bermudez, 32, the mayor of this tiny town on the U.S.-Mexico border, has attempted to outlaw the black market in money that sprang up after the Mexican government devalued the peso and halted trading in the currency.

Now he is seeking support to extend his city's action along the nearly 2,000-mile border, but he is having as little success as King Canute. The money-changers, like the tides, keep coming.

The problem confronting San Luis, a town of about 2,000 people, is common along the border. Since the peso went into its nose dive last year, hundreds or perhaps thousands of currency exchanges have opened.

Some are regular businesses, housed in small buildings along the streets leading from bridges linking the United States and Mexico. But others are somewhat less established, ranging from makeshift tables stacked with money to trucks carrying hand-painted signs posting the daily exchange rate.

It got so bad in San Luis that black market traders were offering bus drivers commissions to bring busloads of people past their stands, while others simply boarded the buses, carrying bags of currency, and began trading.

No one knows how much money has been traded along the border in these businesses, which are not illegal, and it is likely that much of the profit will go unreported to the Internal Revenue Service. But, these transactions are part of the system that has helped the peso find its natural equilibrium in the world market, which today is about 150 pesos to the dollar.

Economists defend this black market. In fact, they do not consider it a black market but part of the overall free market. "There's nothing wrong with it from an economic standpoint," said Leroy Laney, an economist with the Dallas Federal Reserve Bank. "Someone who wants dollars at least can get them."

The exchanges are helpful in two ways. For Mexicans living along the border, they have been perhaps the only source of dollars, which are needed to do business in the United States. For some American businesses along the border, they are a way to recycle the pesos they accept reluctantly from customers in Mexico.

Sergio Hernandez operates a wholesale food business in Calexico, Calif., a border town across from Mexicali, Mexico. A majority of his business is done in Mexico, and since the devaluation he has been forced to accept pesos from customers or face bankruptcy.

But Hernandez says no bank in the United States will convert his pesos into dollars and the Mexican banks do not have enough dollars to make the conversions. So he has taken to the streets, including the streets of San Luis, to sell his pesos to Mexicans who work in the United States and need to turn their dollars into pesos for use in their own country.

"It's a service to my customers and a way for me to stay in business," he said last week.

But Elias Bermudez has a different view. What he sees is a handful of money traders driving down the value of the peso through their speculative currency exchanges. The result, he says he believes, is a further weakening of the Mexican economy.

More importantly, he says he believes that the black market trading has cost established business money in San Luis. Sales tax revenues for the city have fallen more than 50 percent compared with a year ago, and the lower the value of the peso, the less purchasing power of Mexican citizens from San Luis, Mexico, who spend much of their money here.

Last fall the San Luis City Council ruled that only established businesses could trade in pesos and dollars and businesses then voluntarily agreed on a daily exchange rate that was, in effect, trying to prop up the peso at an artificial level.

Local businessmen applauded. "I am 100 percent for it," said Jose Luis Rangel, owner of Save Mart a block north of the border. "The black market is killing us."

But the ordinance simply drove the black market to the outskirts of town, and the council responded by annexing the land they were renting, part of an old flea market.

Things died down a bit early this year when the Mexican government allowed trading in the peso again. But Bermudez said the black market reappeared recently when there were reports that the peso was likely to fall further.

The ordinance took care of all but one trader in San Luis, a man who is working for Sergio Hernandez.

But Bermudez is not satisfied. He has taken his plea to Washington to seek an order from the president regulating currency trading along the border and has written the mayors of scores of border cities asking that they join him.

So far he has no takers, perhaps because they realize the hopelessness of his cause. "People say to me, leave it alone, it will go away," Bermudez said in his town hall office on a dirt street a few blocks north of the border. "But if I do that, I do a disservice to my people."