Diplomats from the United States, the Soviet Union and 11 other nations neared agreement today on the first step toward an orderly exploitation of Antarctica's fabulous wealth.

According to authoritative sources here, the 13 expect to emerge from their closed-door meeting next week to issue a call for an international code governing Antarctica's enormous fish potential. Experts believe there is enough krill, a small shrimp, in the icy continent's seas to double the world's yearly edible fish supply.

The breakthough, expected next week, would set a remarkable precedent. It would mark the first time nations have joinly attempted to exploit the resources of a virgin land with an eye towards preservation and conservation of natural resources.

Most of those at the meeting reportedly agree that the code or convention should be drawn up quickly, probably next year, Chile and France, however, are said to be holding out.

Both claim territory in Antarctica. Chile is said to fear that the convention might threaten its further claim to a 200-mile sea zone. France is thought to be worried by any precedent that enlarges the club, now limited to the 12 original signers of the Antarctic treaty and Poland. A convention would open to all nations.

Diplomats here say there is a common belief that action can no longer be delayed, and expect that the two holdouts will come into line.

Japan, Poland and the Soviet Union are already fishing for krill, and the West Germans have sent out experimental trawlers. So far, the amounts taken have been small. There are fears, however, that if no controls are established, even the abundant krill will suffer the fate of other stocks that have been virtually wiped out in the North Sea, the Mediterranean and elsewhere.

The convention would blind all nations to limit their catch. It would also set up machinery to collect and analyze data on the Antarctic's fish population. Once this is done, quotas could be set for each fishing nation.

Until now, the interest in Antarctica has been almost exclusively scientific. Apart from Chile and France, two countries claiming territory there include Britain, Argentina, Australia, New Zealand and Norway. The United States and the Soviet Union refuse to recognize any claims. The Antarctic treaty, in the best diplomatic fashion, put the issue to one side, where it could stay there as long as there was no wealth to tap. Now there is a question whether this status quo can be maintained.

To complicate matters, U. S. geologists estimate that there are tens of billions of barrels of oil off the continent's shores. There is wide agreement that the frozen land contains gold, uranium, platinum and other minerals.

The meeting here is almost certain to do nothing about any of this, and will probably recommend "further study" of mineral exploitation. The Soviets fear that offshore oil drilling will endanger the krill, and Britain is worried that new finds will weaken the price of its North Sea oil.

The United States had been in favor of drilling as soon as possible, but Washington has shifted to support the view that oil is a scarce commodity.

Nobody thinks the minerals on shore can be mined soon, if ever. Most lie under a mile or more of Ice.

Since krill is already being taken from the waters, the nations meeting here recognize that some form of agreement on fish must now be reached. The others taking part in the talks are Belgium and South Africa.