When the local market for pecans was opened to imports Jan. 1, U.S. growers probably thought they had a chance even with a 40 percent tariff.

But at least three shipments of American pecans have been refused clearance by Korean authorities, and it doesn't look as if they'll be allowed in soon.Ostensibly, the blame rests on the tiny shoulders of the codling moth. According to Webster's dictionary, the larvae of this moth thrive in apples, pears and English walnuts.

Washington contends the pecan tree is not a known host of the codling moth. Even if it were, Washington adds, fumigation could eliminate concern along with the larvae.

This case illustrates how Korean markets are open in theory only in some instances. Import regulations, the Agriculture Ministry's phyto-sanitary ones in the case of pecans, keep the markets effectively closed.

"What we're seeing is that the first line of defense has been peeled away by the liberalization of the market," said one Western trade official who asked not to be identified.

"These other barriers existed, but we're learning more about their depth now because the market is liberalized," he told The Journal of Commerce.

Cherries are another example. Imports of this fruit were officially liberalized in 1985. But it was not until 1989 that U.S. cherries actually appeared in Korean shops. The reason: concerns about infestations of the lowly codling moth.

For years, Washington had prodded South Korea to establish acceptable fumigation requirements. The cherry issue wasn't settled until it had been elevated to the level of trade policy.

The problem with South Korea's markets is not always one of simple non- tariff barriers, one Australian trade official said. "The difficulty is drawing the line between legitimate concerns on health and the gray areas where a country may have another agenda," he said.

New Zealand Ambassador Christopher Butler noted that most emerging countries streamline their import systems overtime.

Even so, South Korea has not willingly opened its agricultural markets. The issue is controversial domestically, with farmers lobbying intensively to keep markets closed as long as possible to protect their own industry.

Last month the National Agricultural Cooperative Federation month urged the Health Ministry to adopt even stronger health regulations on imported food products.

The farmers seem to have strong public support. A recent survey of 1,000 people aged 20 or above found that 86.8 percent opposed imports of farm and livestock products. However, it found Koreans to be less opposed to the general concept of import liberalization.

While 40.6 percent opposed the government's recent easier policies, 22.9 percent favored them with 36.5 percent undecided.

The survey was commissioned by a local business newspaper.

To avoid the label of non-tariff barrier, regulations for imported products should be uniformly applied to domestic products and should be announced early enough so that compliance is possible, said the Australian trade official, who did not want to be named.

An appeal system also makes import restrictions more credible, he said. ''But in every country there is always room for administrative discretion," he added.

Still, the ultimate proof of whether a market is truly open to a foreign product is the presence of that product on the market.

There are no imported pecans in South Korea.