He owns a 45-ton, 3-mast yacht, takes a helicopter to the ski slopes, travels in a private Mystere jet, runs his own soccer team, has chalets, villas and apartments scattered around the world, and is generally thought to be Italy's richest man.

So when it was announced this summer that in 1975 Fiat chief Gianni Agnelli had declared a taxable income of only $53,000, falling from first to 114th place on the list of Turin taxpayers, thousands of Italianeye-brows were raised, the Turin City Council met in special session, and the local tax council decided to investigate.

It turned out that Agnelli had done nothing more than take advantage of an Italian tax loophole that allowsstockholders to pay a flat 30 percent tax on their dividends and omit that money on their personal income tax returns.

With rampant tax evasion now considered one of Italy's most pressing economic and social problems, the sharp reaction came as no surprise.

Turin's Communist Mayor Diego Novelli complained that in an industrialized city where thousands of Fiat workers have their taxes deducted from their paychecks, "It is a question of good taste and style."

The hullabaloo once again focused attention on what has become a sore point both for Italy's taxpaying salaried workers, who make up 81 percent of the labor force, and for the minority Christian Democrat government headed by Giulio Andreotti, which has promised to put a stop to what some have described as an Italian national tradition of dodging taxes.

In fact, after centuries of foreign domination by invaders demanding tribute from local populations, the Italian hatred of taxation is intense. "Better a dead man in the house than a tax collector at the door," goes a popular proverb. Meager pensions, inadequate public services, and a history of widespread government corruption have left many Italians determined to keep their earnings for themself.

There is no accurate estimate of just how much money Italians have succeeded in keeping out of government coffers in recent years, but income tax cheating, illegal exportation of capital, and avoidance of complicated value-added tax system and other direct taxes are thought of the cost the government billions of dollars.

"I never declare everything, and hardly anyone I know does so," said one Roman lawyer. Income tax cheating is thus expected behavior, and there was great skepticism when a Rome newspaper published the 1975 declarations of several well-know persons. Among them were Andreotti, $41,000; Communist Party leader Enrico Berlinguer, $6,800; Guilio Carlo Argan, art historian and Communist mayor of Rome, $68,000; painter Girgi de Chirico, $13,100, and international jeweler Gianni Bulgari, $17,500.

Cheating is so widespread that tax officials rarely believe even those who have filled out their returns honestly, but it is believed to have diminished somewhat since 1974, when a reform bill established the country's first payroll deduction system.

Today, Finance Ministry officials believe that most of the cheating is done by industrialists and professionals such as doctors, lawyers and architects. A recent survey in Rome showed that in 1976 only three doctors declared earnings of more than $45,000; 1,500 declared less than $2,500, 5,000 between $2,500 and $15,000, and 3,000 between $15,000 and $45,000.

"Everyone knows there are hundreds of doctors who take in $45,000 in one month," a surgeon here exclaimed.

Italians have found all sorts of ways to save their tax money. For example, at least a million owners of color television sets appear to have neglected paying the $70 annual subscription fee charged by Italy's state television network.

Other, more wealthy, Italians fly foreign flags on their yachts so as to avoid paying sales and customs taxes, or like film producer Carlo Ponti try to deposit as much of their money as possible in foreign banks.

Ponti, now a French citizen , and his wife, actress Sophia Loren, are being tried in absentia on charges that they illegable exported more than $25 million by setting up fictitious companies.

Most of the cheating done today, however - by manufacturers, importers, shopkeepers, professionals and consumers alike - involves the value-added tax (VAT) introduced in 1973 that oftens runs as high as 30 percent for luxury goods and services.

The government, which only recently established a social security number system that is supposed to facilitate cross-checking, is planning to introduce controls that it hopes will be more efficient.