You must look hard to find an import on the streets of Seoul. The private cars, trucks and smoke-belching buses that crowd alleys and avenues in this city of 9 million people almost all bear the emblems of South Korean manufacturers.

A small but remarkable motor vehicle industry has emerged in South Korea in the past decade, protected by a virtual ban on purchases from abroad. Last year, 221,000 vehicles -- 122,000 of them passenger cars -- rolled off its assembly lines.

With firm control of the home market, the industry is looking abroad, particularly to the United States. Its two significant auto producers, Hyundai Motor Co. and Daewoo Motor Co., are more than doubling their plants' capacity with exports in mind.

Small numbers of South Korea's pioneer passenger car, the tiny Hyundai Pony, already have appeared on the roads of close to 60 countries, ranging from Saudi Arabia to Ecuador to the United Kingdom.

But it has had mixed success, and Korean producers want to crack the world's biggest market.

"You're going to see Korean cars in the United States," an economic analyst at a Western embassy in Seoul said, "but I don't think it's going to be an invasion."

The first cars are expected to reach the United States in 1986.

South Korea has sold immense quantities of steel, textiles and electronics to the United States. But similar success in cars is by no means assured. Formidable obstacles in quality, marketing, protectionism and production capacity seem likely to keep the numbers down.

Even if every one of the 540,000 cars South Korea soon will be capable of producing were shipped to the United States, that still would be less than one-third of the Japanese auto makers' U.S. sales. Still, by Korean standards, the numbers are enormous.

Korea's auto makers hope to take advantage of the American companies' inability to produce subcompacts profitably by helping to fill that niche with their products. They expect to benefit from some special problems confronting Japan, where product costs are rising and exports to the United States are restricted by a voluntary auto agreement. Detroit already is looking to Korea for small models to import and sell as its own: General Motors Corp. has signed an import deal with Daewoo to bring in a Korean-made version of GM's Opel some time after the fall of 1985.

Many South Korean officials are reluctant to discuss their industry's potential in the United States. Hyundai, in fact, continues to maintain that no firm decision has been made to enter the market there at all.

Officials here apparently fear that publicity about their auto programs would prompt a preemptive strike by U.S. legislators before Korean exports get moving. The normally publicity-hungry officials at Daewoo refused repeated requests for an interview.

"We are not a second Japan," said Whee Mok Roh, an automotive analyst at the Korea Institute for Economics and Technology. Some Koreans, in fact, accuse the Japanese, who are wary of Korean competition, of spreading scare stories about waves of Korean automobiles at U.S. ports.

South Korea's much-publicized export miracle began in the 1960s with simple goods such as textiles and shoes. Like Japan, it systematically has diversified export industries in ensuing years toward higher-value-added goods.

Automobiles are seen as ideal next-generation exports. Their production requires mid-level skilled labor and steel -- which South Korea has in great supply. "We want to change the items that South Korea exports in the 1980s," Whee said.

Hyundai and Daewoo -- both of which are members of large, multifaceted industrial groups -- now are investing a total of close to $1 billion in new plant and models, with the approval of the government, which closely directs -- sometimes by force -- the development of export industries. Hyundai is developing a front-wheel-drive car to meet U.S. safety and emissions standards (the Pony does not). The company's one assembly plant, at Ulsan, can turn out 150,000 cars a year, but is being enlarged to a 300,000-unit capacity, according to Hyundai Vice President S. W. Chon. The expansion reportedly will cost $460 million.

Daewoo Motors, a 50-50 joint venture between the Daewoo Group and General Motors, is expanding its single assembly plant in a Seoul suburb from an 80,000-unit capacity to 240,000 at a cost of $500 million. It will introduce a front-wheel-drive "world car," modeled on the Opel, to be sold both at home and abroad.

Chrysler Corp., meanwhile, also is scouting for an offshore subcompact. It reportedly has been talking with Samung, a South Korean conglomerate that never has produced cars, about jointly turning out 100,000 cars a year for the United States. Officials at both companies say no final deal has been signed, however, and it is unclear if the government would allow a third manufacturer into the market.

South Korea began assembling vehicles under license to foreign companies in the 1950s. But it passed a milestone in 1974 when Hyundai introduced the Pony, the first car designed and engineered in South Korea.

Today, Hyundai controls 75 percent of the domestic market, and Ponies lined up six and seven deep at traffic lights and taxi stands -- now joined by a sportier Pony II -- are a common sight in Seoul. Daewoo offers half a dozen models and holds all but a few units of the remaining market.

After rough times in the early 1980s, as South Korea's economy suffered serious dislocation, the motor vehicle industry is on the rebound. Car production rose from 69,000 units in 1981 to 122,000 last year. Bus and truck production recorded major increases, too.

Still, automobiles remain beyond the reach of most South Koreans. Producer and owner are subject to more than 20 special taxes and levies, including compulsory purchase of subway construction bonds, a defense tax and a quarterly road tax. Gasoline costs more than $4 a gallon.

"The people who buy cars are still the rich," Hyundai's Chon said. These taxes generate revenue, but also, in the Seoul government's eyes, serve a social function of curbing consumption and oil imports.

Exports began in 1976, when Hyundai sent five Ponies to Ecuador. Since then, Hyundai claims to have sold cars in more than 60 countries in Africa, the Middle East, South America and Western Europe, virtually every country that would allow foreign-built cars through Customs.

Daewoo also is exporting a small numbers of cars now. But Korean cars have failed to catch on in many markets, and exports have slipped from 17,200 in 1981 to 14,100 in 1982. Korean officials mention the desire of developing countries to build their own cars; critics mention problems with the Korean cars' quality and the lack of higher-line offerings.

Exports began growing again in 1983, reaching 16,400 units, with about half of them sent to Western Europe. Last year, Hyundai also began to sell the Pony II in Canada, an apparent test for the U.S. market. So far, just over 12,000 cars have been registered there, sold and serviced by about 100 dealers.

The Pony retails for as little as $4,500, and it has been called the lowest-priced car available in Canada. Indeed, Korean manufacturers' low production costs may prove to be their most important advantage.

Hyundai's assembly-line workers start at about $2 an hour, roughly one-tenth of the hourly wage and benefit costs for U.S. auto workers. Whee of the Korea Institute for Economics and Technology estimates that the Korean industry has a $1,500-per-unit cost advantage over the Japanese, although retail prices suggest it is less than that. (The Japanese, in turn, often are estimated to have a $1,500 advantage over their U.S. competitors.)

The South Korean government has helped by designating automobile manufacturing a "strategic export industry." That means that, like other export operations, the auto makers get priority access to South Korea's limited credit facilities and other export incentives. But according to Whee, there is no special incentive aimed at automobiles alone.

Quality is among the Korean cars' big drawbacks. From its inception 10 years ago, the Pony has been the subject of stories of glove compartments that don't close and plastics that melt under a hot sun. Hyundai's Chon acknowledged minor problems, but said major components such as the drive train are sound.

Korean cars continue to trail U.S. and Japanese models in fuel efficiency, integrated circuitry and sophistication of some steel components, according to diplomats who follow the industry here. A Pony with a 1.2-liter, 80-horsepower engine gets about 31 miles per gallon. Korean officials say the new models will correct many of these complaints.

Technology on the factory floor also lags -- 10 years behind the United States and Japan by Chon's estimate. Hyundai, owned 10 percent by Mitsubishi of Japan, is drawing on its foreign partner for help. But Daewoo, with a closer link to GM, seems better positioned to bring in modern technology.

Styling has been cited as another drawback. The first Ponies were little more than metal boxes on wheels, lacking aesthetic identity. But in recent years, Daewoo and Hyundai have come out with sleek models that at first glance are indistinguishable from Japanese or U.S. ones. Some of that is in preparation for expected liberalization of import restrictions in 1988 that will allow foreign models into Korea.

More fundamental questions address the maturity of the industry. Taxes and low personal income have hampered development of a large and stable home market that would help producers weather fluctuations overseas, it is argued. Although major rises in domestic sales are forecast, the home market could remain a problem.

In addition, suppliers tend to be small, back-yard operations. According to one Western embassy's analysis, 70 percent of the 385 firms in the parts business were capitalized at less than $630. Parts quality is a problem, too, with foreign companies unwilling to buy parts for use in their own production.

Even if Hyundai and Daewoo can offer competitive models, they face a serious marketing challenge in the United States, where they have no reputation or sales network. The Japanese sustained years of large investments and heavy losses breaking into the U.S. market in the 1960s.

Daewoo has cast its lot with GM. That would give it greater assurance of high sales volume. But it would not acquire a name among American buyers, and any profits would be shared.

Rumors continue, however, that Hyundai is discussing a similar deal with Ford. But since its inception, Hyundai has had a reputation for going it alone. It is now is talking of direct U.S. sales through a network of dealers and service agents it will build. The company appears to be reasoning that this will ensure that the Hyundai name will become known and the profits will revert to it alone.