Maria, a Cuban girl with tight red slacks, a revealing yellow blouse and modest platform shoes talks to a foreign man near the entrance of the Hotel Nacional here. They wander off together, headed for one of Havana's 70 posadas - motel-like establishments that rent rooms to couples by the hour with no questions asked.

Maria is 17, a school dropout and a self-employed prositute. In any other Caribbean tourist resort or in pre-revolutionary Cuba it would have been a common sight. But in Cuba today it's an anomaly, a break-away from revolutionary-era social norms and a serious violation of Cuban law.

In 1957 about 100,000 prostitutes operated in Cuba. Many of them were even younger than Maria, brought in to the city's red light district by men who ranged the Cuban countryside like baseball scouts. Unlike Maria however, the girls who sought to escape rural poverty became trapped in big-time businesses run by mobsters, pimps and madams.

In 1961 the new Cuban government of Fidel Castro cracked down on organized prostitution and gambling. Casinos and brothels were closed, and Castro urged the pimps and gamblers to go to Miami at Cuban government expense. Those who didn't leave were sent to correntional prison. The prostitutes, who were considered victims of exploitation, were sent to rehabilitation centers and later given jobs.

Since then, prostitution, gambling and drugs have been considered anathema to Cuban revolutionary principles.

The phenomenon of young prostitutes like Maria began with the spurt of foreign tourism in recent years. Somehow, Maria discovered that a few hours with a foreigner could bring her rich rewards. The foreigner has hard currency and access to the dollar stores selling radios, recorders, pretty clothing and other items in an austere economy with strict rationing of food and clothing.

The Cuban Government is keenly aware that foreign tourism extrances the danger that there will be more Marias. More than that, it believes the influx of tourists, and especially Americans who will be coming in droves in future years, will carry the seeds of corruption that thremen to contaminate "revolution morally."

Cuban President Castro sees the problem in strategic terms. In a September speech he said that the CIA's secret war of the 1960s characterized by arms incursions, sabotage and assassination attempts is no longer in vogue. Since that strategy failed to destroy the Cuban revolution, Castro said the strategy now takes the form of "a campaign to erode the revolution ideologically."

Castro recognizes the economic support of tourism.

"We are not an oil producing nation," he said in his September speech. "We have to exploit our sun, sea, and natural beauty."

But Castro laid down the guidelines: "Don't led anybody think that we will permit foreign tourism with gambling, casinos and prostitution. We would be willing to die of hunger before agreeing to that kind of violation of our people's morality and the ethics of the revolution."

The flight to raise political awareness in Cuba visavis foreign tourists has been especially sharp among service and restaurant workers.

The practise of accepting tips and gifts from foreign tourists became a topic of discussion at a recent Congress of the Union of Service and Restaurant Workers. The congress went on record as "categorically rejecting tipping as a corrupting element of our revolutionary customs and humiliating relaxation of our revolutionary ethics."

Tipping had originally been ended by a vote of the workers in 1968 during the so-called "Revolutionary Offensive." Since larger foreign tourist travel to Cuba began in 1972, tipping has made a comeback. Recently a American television cameraman left a large tip at the lavish 1830 Restaurant here in Havana. He was already outside the front door when the head waiter came running to return the tip. However, there have been numerous cases in which the temptation of the tip has been greater than the ethical purity to some waiters.

Castro has stated that to strive for revolutionary purity, "a wall does not have to be built around our island."

"The real defense," he said, "has to be built in our political awareness and the dignity of every Caban."

Despite the recognized risks. Castro exuded confidence during his September speech: "We aren't afraid of visitors because we have confidence in ourselves."