Last summer I visited a woman in Paris for whom I once worked as an au pair. Over lunch in her apartment in the city's elegant 7th arrondissement, Chantal told me that she was about to leave for a trip to Iran with a few female friends, and she described -- with her exquisite French fashion sense -- what she planned to wear. She had bought several long robes, she explained, and they would reach from her neck out to her wrists and down to her ankles, thus shrouding her entire body. When her adult daughter suggested she could show respect for Iran's Muslim culture by covering only her hair with a simple silk scarf, she said, No. Not the slightest piece of neck must show. She planned to conceal everything but her face.

When in Iran, Chantal explained, she would do as the Iranians do.

Not that such accommodations would be expected of visitors to Paris these days, she added, slightly acerbically. Or of visitors to Amsterdam, London or Rome, for that matter. Even many immigrants who now make their homes in those cities, Chantal said, continue to live largely according to their old ways.

She's right, of course. Gone from the immigrant-receiving countries of northern Europe is the tradition of "When in Rome, do as the Romans do." It has been replaced by the ethos of modern multiculturalism, the philosophy of the fruit salad as opposed to the message of the melting pot: "When in Rome, do as you did back home."

Sikhs in Britain have won the right to wear turbans instead of crash helmets on their motorbikes; Muslim students in Germany ask to be excused from coed swimming sessions; Moroccan immigrants to the Netherlands continue to import subservient wives from their homelands, often in arranged marriages. The examples are so commonplace that when France resisted -- and ultimately prohibited -- the wearing of ostentatious religious symbols such as head scarves in schools, the move provoked international debate.

All of which presents particular tensions for the welfare states of northern Europe, whose identity has centered around fostering common ways, common values, common needs. "Sharing and solidarity can conflict with diversity," writes David Goodhart, founding editor of the monthly magazine Prospect, which bills itself as "Britain's intelligent conversation." Many Europeans are left reflecting upon the irony of multiculturalism: It protects and preserves every culture, except one -- the host culture.

That is the challenge European countries have been confronting. While France has openly rejected the ideology of multiculturalism in order to conserve its own culture and constitution, the reality of multiculturalism has arrived in the country anyway -- as those marginalized mobs in the Parisian suburbs demonstrate. Meanwhile, such countries as Britain and the Netherlands, which have openly embraced multicultural ideology, are now recognizing the disheartening reality of growing ghettos of poorly integrated immigrants.

Two different approaches, two distressingly similar outcomes.

The rampant rioting in France has caught the rest of Europe's attention because no country feels immume to unrest. Trevor Phillips, the black chairman of Britain's Committee for Racial Equality, warned recently that his country is sleepwalking toward segregation: "We have allowed tolerance of diversity to harden into the effective isolation of communities," he said. This year, which marked the 40th anniversary of Britain's first anti-discrimination laws, the Race Relations Act, also exposed the alienation of some second-generation Muslims after the July bombings in London's public transport system. And just last month, while I was in England, rival gangs of Afro-Caribbean and South Asian youths clashed in a Birmingham suburb, leaving two dead.

As a result of incidents like these, former advocates of multiculturalism in Britain and the Netherlands are beginning to sound a little like French officials, determined to find ways of making their immigrants "British" or "Dutch"; at the same time, the French are being forced to acknowledge that their bureaucratic attempts to assimilate minorities have failed, and they are beginning to talk about ways of accommodating immigrants' special needs.

Everyone looks across the Atlantic at the world's most ambitious yet imperfect multicultural experiment. Can countries that don't have America's founding philosophy of equality and have not yet suffered the liberating agonies of a civil rights movement find a philosophy that will put their citizens -- new and old -- on equal footing?

"Europe is not America," Goodhart warns. "One of the reasons for the fragmentation and individualism of American life is that it is a vast country. In Europe, with its much higher population density and planning controls, the rules have to be different. We are condemned to share -- the rich cannot ignore the poor, the indigenous cannot ignore the immigrant -- but that does not mean people are always happy to share."

When a culture tries to impose its ideology on immigrants, they resist. The secret of multicultural success may lie instead in encouraging newcomers to engage in the evident vitality and impressive values of the receiving culture.

When in Rome, it would then become a matter of pride to do what the Romans do. And help make Rome a better place than the Romans ever did themselves.

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Frances Stead Sellers, who grew up in Britain and has lived in both France and the Netherlands, is an assistant editor of Outlook.

Finding common cause: Muslim women wore traditional dress when they joined a demonstration in central Paris last year to protest the detention of two French journalists being held hostage in Iraq.