In March, Jean-Marie Le Guen, a Socialist member of Parliament who is a doctor, proposed wide-ranging legislation to combat his country's expanding girth. "We are confronting a major public health problem," said Le Guen, author of "Obesity, the New French Disease," which was just published and is one of many new books on the topic.

Germany, Italy and England are also battling serious obesity problems, but the issue is particularly striking here because, until recently, the slenderness of the French has been so mythic that some scientists theorized that it must have genetic roots.

In fact, in France, as in much of the world, the culprit is changing eating habits, experts said, as France's powerful culture of traditional meals has given way to the pressures of modern life. The French now eat fewer formal meals than they did just a decade ago and they snack more. Another cause is the rising availability in France of fast food and prepared foods, which tend to be higher in fats and calories.

Food companies say that France is one of the most promising international markets for prepared items like frozen pizza, as well as for outlets like McDonald's and Kentucky Fried Chicken, both of which are planning to open dozens of new stores in the country this year. Perhaps predictably, sales of Weight Watchers products, introduced to France in 1992, are also booming; it now sells 3,000 tons of frozen food annually.

Doctors here say that the French are unusually susceptible to the invasion of convenience foods since their culinary traditions insulated them for so long.