For centuries tiny Luxembourg has endured invasions by powerful neighbors, but the latest threat to the grand duchy's survival comes from its own citizens, or lack of them.

A sharply declining birthrate, coupled with swelling ranks of foreign residents, has begun to provoke worried worshippers in political quarters of a slow "national suicide."

Foreigners account for about a quarter of the population, and the proportion is expected to exceed one-third by the year 2000.

Deaths have surpassed births for a decade, a troubling trend in this diminitive state of 357,000 people. Its annual birthrate of 8.5 per 1,000 population, is one of the world's lowest. The U.S. rate is 14.9. Prime Minister Gasion Thorn calls the reluctance to procreate "the gravest problem Luxembourg faces over thelong term."

As in other Western societies, prolonged affluence seems to have blunted the desire for large families. Intent on preserving their high living standards, many couples in Luxebourg decide not to have a third child, which can involve such burdens as moving from an apartment to a more costly house.

Moreover, working women find that bearing more than two children can interfere with their career plans, government officials note.

"In most prosperous countries, an increasing number of women are entering the job market rather than just looking for a life of raising children," said George Als, director of Luxembourg's economic and statistical studies. "But we sense the effects more here because our country is so small."

To encourage larger families, the government is considering tax breaks for couples with three or more children. But most officials are wary about granting outright bonuses for newborn children as Belgium and France do.

"A lot of people here think that might be taking the role of the state a bit too far," one government official said.

The shrinking local population has magnified the need for immigrants. Most are laborers from Southern Europe who come to work in the sprawling ARBED steel mills, but the newcomers also included foreign bureaucrats employed at the seat of the European Parliament.

The alien influx has alarmed some Luxembourgers, who detect a certain loss of local identity with each new Greek, Italian or Turkish restaurant in the neighborhood.

Others welcome the foreign influences, however, believing that the grand duchy should stop guarding so zealously the quaint social traditions that have lampooned in operettes.

Unlike other European countries with large migrant groups, Luxembourg has avoided racial conflict, probably because foreign workers have been a threat in the job market. Unemployment is virtually nonexistent.

"Luxembourg is always short of labor, and in some industries, up to 85 per cent of the workers come from abroad," government spokesman Andre Claude said.

Officials still express concern about the erosion of local culture, however, young people must to go French or German universities, and they often decide to settle abroad.

Even when they return, many young adults "appear less attached to the country," Als observed. "Patriotism is difficult for a small nation, which has less control over its destiny," he said.