Fruit on the bottom, fruit on the top, low fat, higher fat, extra smooth, extra expensive. For breakfast, for lunch or for snacks, all the variations have been thought of by yogurt manufacturers, and the public is buying them. In 1960, according to USDA figures, sales of yogurt were about a quarter of a pound per person. By 1984, that figure had jumped to about 3 1/2 pounds per person.

The nutritional benefits of yogurt, whether for lactose intolerant individuals (people who have trouble drinking milk can more easily digest yogurt) or as an excellent source of protein and calcium (a cup of yogurt contains about 300 milligrams of calcium, the same as a glass of milk) are no doubt part of this boon.

And although it is premature to make health claims about yogurt, according to Manfred Kroger, professor of food science at Penn State University, it has a "lot of placebo power," he said. (Some studies have shown a possible cholesterol-lowering effect from it and researchers at the USDA's labs in Beltsville have found that rats on yogurt-supplemented diets grow faster and are better able to resist disease.)

Yet what is otherwise a nutritious food is often misconstrued to be a diet food. Plain lowfat yogurt -- a purist's delight -- has been infused with extra fat, sugar and sugar-coated fruits that can add up to 260 calories per cup for some fruit-flavored yogurts.

But just what are the differences among yogurts besides their consistencies, flavors and pretty packages? One difference is the amount of fat. According to the Food and Drug Administration, whole-milk yogurts must be over 3.25 percent butterfat. Lowfat yogurts must contain between 0.5 and 2 percent butterfat.

In whole-milk plain yogurt, says Kroger, fat is responsible for half the calories. In skim-milk plain yogurt, none of the calories come from fat; they come from the milk sugar and milk protein. Lowfat plain yogurt comes somewhere in between; about a third of its calories come from fat.

As indicated in the accompanying chart, more fat and calories sometimes mean more money. Whitney's, for example, tops the charts in cost at 13 cents per ounce (twice as expensive as Giant and Safeway's Lucerne), 33 calories per ounce and ties with Colombo in having 23 percent of its calories come from fat. (In addition, it is the only yogurt in the bunch to contain egg yolks.) Kroger said, however, that if a yogurt is more expensive, it may mean that the fruit is costlier or that more has been used.

Lowfat Dannon, on the other hand, has the lowest percentage of calories from fat, although the highest percentage of carbohydrates per ounce.

Aside from corn sweeteners and cane sugar, fruit pure'es and preserves listed on the label of a fruit yogurt contain approximately 50 percent sugar themselves, according to Kroger. A good rule of thumb is that fruit yogurt contains anywhere from 20 to 30 percent fruit mixture, he said.

However, it is impossible to tell just how much sugar has been added to yogurt -- the ingredient label does not indicate the specific quantities, only their relative amounts, and manufacturers are hesitant to divulge the information. Additionally, the carbohydrate count includes fruit, lactose in the milk and sugar.

Other observations from the chart: Giant and Safeway's Lucerne yogurts stack up almost identically in terms of cost, calories and so on. And although yogurt is a low-sodium food, there is some difference in sodium because of the quantity of nonfat dried milk added.

Other tips for reading yogurt labels: Ingredients such as guar gum, modified cornstarch, gelatin or agar agar are stabilizers in yogurt; they prevent separation of whey, a substance that is a by-product of yogurt making, acccording to Kroger. And today, most yogurts contain "live active cultures," although the industry has the option of pasteurizing the yogurt after incubation, a technique that destroys the beneficial bacteria but prolongs shelf life, Kroger said.