ew financial industry rules could threaten the growth of one of the most vibrant drivers of the Internet's early success: naughtiness.

In the wake of rules from credit card companies and banks that have strangled many online gambling sites, Visa and MasterCard are now looping the noose for adult sites that may have spotty credit-card records. Many of the online companies say that the new rules, which the card companies call antifraud measures, will clean up an industry rife with unethical billing practices. But some operators say that, in fact, the credit card companies have taken it upon themselves to step in as de facto regulators of their industries.

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"It gives Visa a level of control over the way business is done," said Tom Hymes, the editor of AVN Online, a magazine that covers the online adult industry. "The smoke signals are worrisome."

Others say that the costs, while modest, could also drive out some of the smaller sites that have very small profit margins.

The financial companies say the rules merely extend to a new group of businesses policies that have long been in place for combating fraud. "These are wild-eyed, crazy theories," said Martin Elliott, director of corporate risk at Visa, of the idea that this is the beginning of a larger crackdown on adult fare. "We're just trying to protect our payment system and cardholders."

At issue are rules and fees that went into effect on Friday — measures that apply to sites and companies that the credit-card issuers call "high risk." Visa will charge its member banks a $500 registration fee and an annual renewal fee of $250 for each high-risk company they pay a credit-card charge to. Those fees will be passed on, with a markup, to the sites themselves. MasterCard is expected to roll out similar fees, industry analysts say.

The sums are insignificant to larger sites, but could well drive smaller sites into the red; tens of thousands of adult sites are home-grown entities, industry experts say.

To Mr. Hymes, the magazine editor, it all looks like an attempt to put a broader squeeze on the industry. He said he was especially worried by a Visa statement on the fees that has been posted to his company's Web site: "These steps will also help keep illegal activity from entering the Visa network." That, he says, could mark the beginning of decisions by credit card companies based on the content of sites. "The implications of that statement are really chilling," he said.

Many of the entrepreneurs who run such sites agree. "I'm concerned that Visa or MasterCard could use their position to regulate content on adult sites," said Brooks Talley, who runs a site devoted to bondage and sadomasochism through his company, the FRNK Technology Group.

The new fee structure comes at a time when banks that issue MasterCard and Visa cards have already made a significant impact on online gambling. Numerous banks, including some of the nation's largest, now entirely prohibit the use of their cards for online gambling. The banks say that they are not sure that Internet gambling is legal, and that they do not know if they will be repaid for extending credit when some courts have ruled gambling debts are unenforceable.

PayPal, the big online payments company, which acts as an intermediary for consumers to buy from Internet merchants using their bank accounts or credit cards, has also said it would no longer accept payments from gambling sites. Some of the 2,000 sites devoted to gambling have said the added fees from Visa and MasterCard alone have caused their revenues to drop by as much as 70 percent.

The credit-card companies say that the new charges for adult sites are necessary because those sites cost them more money in fraud and a practice known as "chargebacks." In a chargeback, a credit card holder denies having made a purchase and demands that the company take the charge off the bill. This practice, which gives new depth to the term "buyer's remorse," often occurs, for example, when a husband incurs a charge and his wife discovers it, said Chris Mallick, the chief executive of Paycom Billing Services, another financial intermediary company.