Take a well-promoted product category in which all brands seem the same, change the packaging, increase the price, advertise it and give it a new name. What do you have? An instant success. "This is the type of product success tha t is definitely a milestone in one's career," said Paris-based Yoplait marketing manager Bruno Duberga, 35, recalling the early 1995 launch of the very French treat, Petit Filous aux Fruits "A sucer." Yoplait's breakthrough bran d-chilled fromage frais, or yogurtlike cheese, packaged in small plastic tubes for kids-was almost constantly out of stock during its introductory year, forcing Yoplait to postpone advertising and scramble to double production. Tod ay, Petit Filous aux Fruits claims 6% of the petit suisse (fromage frais for children) market in value and 4% in volume. It also has propelled the once-flat $274 million sales category to grow by 4% in 1996, according to the compan y, as several competitors have launched imitations. Buoyed by the French success, Yoplait marketers in Spain, Portugal, the U.K. and the Benelux countries have begun offering the new product while Italy is testing it.Of all the mar keting components, packaging has been the most important contributor to the success of Petit Filous. Before the product hit the market, kids ate their fromage frais with a spoon at the kitchen table. But the soft plastic tube that Petit Filous comes in changed that-courtesy of Yoplait, which spent two years in research and development and invested $588,600 in a custom-made machine that formats, fills and seals soft packages. Although applications for the tec hnology exist in the cosmestics and pharmaceuticals industries, Yoplait was the first food group to adopt it.Kids can now suck Petit Filous from the tube through an easy open seal and take the treat with them. Petit Filous also can be frozen into an ice cream-like product and consumed on a wooden stick (wooden sticks are included in each pack). With the packaging set, Mr. Duberga positioned the brand as a nutritional, easy-to-use spoonless treat that mothers could feed their 4- to 8-year-olds any time, anywhere. "We were not interested in a product which would be limited to mere switching," said Mr. Duberga. "We wanted a product which would increase frequency of usage." Yoplait's new product, retailing at $2.35, comes in packs of three units, each with three flavors. To get consumers to try the new brand, Mr. Duberga opted for brand image advertising rather than price promotion, even though discounting is a favored tactic among marketers in this category. He hired the Paris office of McCann-Erickson Worldwide to support the Yoplait launch with a $1.9 million TV ad campaign demonstrating that kids fed up with being small will favor li cking a Petit Filous in lieu of a thumb. "With such a strong concept, TV is a must," said Mr. Duberga. "It is the best medium to induce the buying process. The goal is to have consumers trying the product." Indeed, samplers (an d their moms) have come back for more. The repeat purchase rate is 50.%. Next year, Mr. Duberga aims to get more consumers to try Petit Filous by increasing the ad budget and launching a new TV campaign. New flavors are also planne d, as are promotional campaigns. "Our buy-again rate is good," he remarked. "We now have to improve and increase our penetration rate."And what about competitors trying to mimic Yoplait? "We expect competition sooner or later, " Mr. Duberga said. "Our best protection will be to work hard on consumer satisfaction."