THERE'S A LOT to choose from when it comes to buying fresh fruit and vegetables in Washington. And the picking is not only in the produce. Farmers' markets--springing up all over Maryland, Virginia and the District--add to the hundreds of roadside stands and private vendors' markets specializing in fresh local produce at reasonable prices. In these markets shopping is leisurely, and hopefully careful, as it pays to shop from truck to truck, and vendor to vendor for the best prices and quality. It can't be too leisurely, though, because the produce tends to run out by noon. Supermarkets, on the other hand, specialize in convenience, seemingly endless supplies and low prices on bulk items.

The first local white corn, tomatoes, melons, peaches and limas should fill the marketplace this week, according to local extension offices. So we took a survey June 30, July 1 and July 2, making a visit to three of each market category: roadside stands, private vendors' markets, open-air farmers' markets and supermarkets to discover what consumers can look forward to this year.

Quality promises to be high, prices low--juicy raspberries $1.50 a pint, firm green beans 49 cents a pound, tiny squash 5 pounds for $1 and purple-red cherries 59 cents a pound. Farmers' markets, private vendors and roadside stands didn't come close to matching supermarkets' special prices on certain items, but in most instances what the alternative markets sold from local sources was priced on their par if not better. Watch claims that the produce is local. Several vendors made claims about having Virginia Silver Queen corn for sale, but it won't be coming out of the ground until this week, say Fairfax County extension agents. Open-Air Farmers' Markets

Six of these markets have surged into the marketplace in the last decade, one in every major county in Maryland, Virginia and the District. The rules are simple: farmers pay a minimal fee to park their trucks on city- or county-owned parking lots and sell farm products, at least 80 percent of which come from their own farms.

Their produce is generally the freshest in the marketplace, because farmers don't pick their crop until the day before coming to town. In addition, they eliminate the cost of the middleman by bringing their produce directly from the farm to the markets; therefore everyday prices for good local produce are likely to be low. As prices vary within these farmers' markets, it pays to shop around once you are there. At the D.C. Open-Air Farmers' Market, for example, zucchini ran 3 pounds for $1 at one stand; 5 pounds for $1 at another. At the Arlington Farmers' Market zucchini cost from 25 cents a pound to 65 cents a pound; green beans there ranged from 60 to 75 cents a pound.

Least expensive was the D.C. market at RFK stadium. Since this is the largest market, with over 35 farmers, price competition runs high. Green beans, slightly softened by the afternoon heat, averaged 49 cents a pound; raspberries $1.50 a pint. First-of-the-season tomatoes averaged 69 cents a pound.

The Prince Georges County market, the second largest market in the survey, had generally fresher looking fruit and vegetables than the D.C. market on the day of our visit. Here fresh, crisp-looking iceberg lettuce sold for 65 cents a head and the last of the season's tiny, ripe strawberries were 75 cents a quart. They had some of the few blueberries around, and the peaches were soft and ready to eat.

Service at the Arlington Farmers' Market was exceptionally friendly. But produce was a little skimpy, as most of the farmers fill their stands with their own crops and their crops were still another week away from harvest. The best buy there was the tiny, early corn at 20 cents an ear. Raspberries were fresh and well priced at $3 a pint. Chili peppers were three for 25 cents, and this market had the only tiny white eggplant in town. Private Vendors

Private vendors, either individuals or groups who come together in one location, don't necessarily grow the goods they sell themselves, but they do stock their stands with fresh local produce. Their prices usually run a little higher than farmers' markets. Variety is good and you'll find most anything in them.

The Bethesda farm women's market (officially known as the Montgomery County Farm Women's Cooperative Market) is fun to shop in, as you can spend as much time outside the tiny white one-story building among the antique furniture and dishes as you can inside among the fruits and vegetables, chickens and hand-packed sausage links. Green beans were high--89 cents a pound; cherries were $1.85 a quart, the second highest price we found for them.

Prices were competitive at the Eastern Market. Floros J. Soiles, a vendor with a tiny stand at this old warehouse, had the prettiest display of the survey and among the highest prices, even though they fluctuated dramatically. Cherries $1.89 a pound one day dropped to $1.59 a pound the next. Still they were high for the survey. Raspberries were fat and pricey at $2.90 for a half-pint. Just next to him Calomaris & Sons was selling raspberries for $2.79 a pint, but his cherries were still high at .98 a pound.

The Silver Spring Market, where farmers set up tiny tables in the Silver Spring Armory parking lot, is closer in concept to the county-run farmers markets. Lettuce was the best buy of the day here--fresh and crisp and 40 cents for a 1-pound bag of green leaf lettuce; red leaf ran 75 cents for a 1-pound bag and iceberg was 55 cents a head. There was kohlrabi for 20 cents a pound, and customers were grabbing up sugar snap peas at $1.25 a pound faster than the vendors could fill the pint baskets they used for display. Roadside Stands

Roadside stands are numerous, practically one in every neighborhood once the season is full-blown. They range in design from a quick tailgate display to fixed sites with doors to lock. Variety and the prices are competitive with the private vendors.

Quality runs high at Mrs. Beall's setup on picnic tables in McLean where you can smell the canteloupe from your car. Crisp green beans were 89 cents a pound, as were tiny yellow squash and zucchini. There were some local tomatoes at 98 cents a pound, but they were gone by 10 a.m. Baby pickling cucumbers were 89 cents a pound.

Much of what was displayed at Coleman's Market in Annandale had passed its prime the day of the survey, but variety was great. Greens were withered, yellow squash was wrinkling, radishes were soft and tiny. It had the prettiest watermelons around, and the Chinese cabbages, coriander and bean sprouts looked crisp and fresh.

Serio's in Rockville had exceptionally good prices on produce. Four types of crisp, dark green lettuce ranged from 69 cents to 99 cents a pound; green beans were 49 cents a pound; cherries were 79 cents a pound. Supermarkets

In grocery stores you get convenience, variety and unbeatable prices on bulk items. Beautiful bing cherries, for example, at the Georgetown Safeway, Magruders in Rockville and Giant Food in Falls Church were priced well below any at the other outlets. In addition, produce comes from across the continent--even the world --making variety the widest in supermarkets. Peaches, cantaloupes and honeydew melons reach their displays long before nearby farmers get them out of the ground or before vendors can afford to pay high prices for their smaller orders. There were at least five different hot pepper selections at the Falls Church Giant Food and the Georgetown Safeway. Even so, none of the supermarkets had fresh raspberries in stock.

The produce sections at the Fall's Church Giant Food and Magruder's in Rockville looked sloppy; pieces of produce were falling off the displays and some sent one customer skidding across the floor at Magruder's. Customers complained out loud about the unripened cantaloupes at the Giant Food and the withered romaine at Magruder's. Green beans were soft, but well priced (3 pounds for $1 at Magruder's; 2 pounds for 69 cents at Giant Food). Squash was fresh and ripe at both stores, with good variety.

True to its reputation, Magruder's had the best prices. Zucchini 4 pounds for $1; georgous ripe cherries at 59 cents a pound; watermelons 5 pounds for $1; unwaxed cucumbers 5 pounds for $1.

The Georgetown Safeway, on the other hand, had some of the highest prices and the prettiest displays in the supermarket category. Produce looked well cared for and plentiful. Green beans were pricey at 99 cents a pound, but firm. There were five types of crisp-looking lettuce ranging from romaine at 39 cents a pound to red leaf at $1.49 a pound; the crenshaw, cantaloupes and honeydews were ready for eating and the biggest we found at a supermarket.

The following chart (on Page E24) represents the survey's findings. Prices quoted for farmers' markets and the private vendors' markets represent the range in prices found that day within the specific market.