TOKYO, DEC. 27 -- Japan's census workers tallied a total population of 123,611,541 as of Oct. 1, 1990, meaning that this crowded nation remains the seventh most populous on Earth.

But the population increase of 2.1 percent since the last national census, in 1985, was the country's lowest rate of growth since well before World War II. That reflects Japan's plummeting birth rate, a social statistic that the government has labeled a major national crisis.

The preliminary census figures released this week document what every subway rider here knows already: More and more Japanese people are crowding into urban centers, such as Tokyo, Yokohama and Osaka, while rural areas are experiencing population declines.

The census takers computed a population density for Japan of about 1.4 people per acre, more than 10 times the density of the United States. In practice, population density here is even greater because only about a quarter of Japan's rugged, mountainous land mass is habitable.

The census takers who walked the streets and apartment house corridors of Japan last fall were given special forms and instructions for counting foreign residents.

While final numbers are not available yet, the federal Management and Coordination Agency said the population total includes about 900,000 foreigners. That would be the largest number of non-Japanese voluntarily residing here in recorded history. Just before and during World War II, there were about 1.3 million foreign residents in the country, but they were mostly laborers brought here forcibly from the Asian nations Japan colonized in the first half of this century.

The new census data mean that in total population, Japan holds its global rank behind China, India, the U.S.S.R., the United States, Indonesia and Brazil.

In terms of total wealth, as measured by gross national product, Japan ranks as the world's second richest country, behind the United States. With a population just under half that of the United States, Japan's GNP of $2.9 trillion -- the Bank of Japan's estimate -- is about 60 percent as big as the U.S. GNP.

On a per capita basis, Japan's GNP is the world's highest -- about $23,400. The United States ranks second in this category, with a per-capita GNP of $19,800.