April 20, 2004 - The number of foreign-born residents and children of immigrants in the United States has reached the highest level in history, according to a Census Bureau report released yesterday. It found that the number had leapt to 56 million from 34 million in the last three decades.

Mexico accounted for more than a quarter of all the foreign-born residents, the bureau's analysis of data from its March 2000 Current Population Survey showed. That share is the largest any country has held since the 1890 census, when about 30 percent of the country's foreign-born population was from Germany.

The study found that, on average, foreign-born residents were much more likely than native Americans to live in or around a handful of big cities. They were almost equally likely to be in the labor force. But foreign-born residents earned less and were less likely to have health insurance than native Americans.

While the number of foreign-born residents and their children is higher than ever, their percentage in the population is not. In the 1910 census, that group made up 35 percent of the population, compared with 20 percent in 2000, a spokesman for the Census Bureau said.

The report brings together data on the age, sex and birthplaces of the foreign born, their education levels, jobs and earnings. In doing so, it makes clear the near impossibility of generalizing about immigrants and the immigrant experience.

For example, while only 33.8 percent of residents over age 25 and born in Mexico had completed high school, 95 percent of those born in Africa had. While the median household income for those born in Latin America was $29,338, it was $51,363 for those from Asia, well above that of native Americans.

The proportion of married couples with children under 18 ranged from 35 percent for residents born in Europe to 73.4 percent for those from Latin America. The proportion of naturalized citizens varied widely, from 52 percent of those born in Europe to 21.1 percent of those born in Central America.

"The big question is how the second generation is going to do," said Nancy Foner, a professor of anthropology at the State University of New York at Purchase and the author of "From Ellis Island to JFK: New York's Two Great Waves of Immigration" (Yale, 2000). "And how the presence of large proportions of Asians, Hispanics and black immigrants are changing Americans' notions of race."

The Census Bureau study found that the foreign-born population was heavily concentrated in California, New York, Florida, Texas, New Jersey and Illinois; those six states accounted for 70.4 percent of the total. Nearly 55 percent lived in the nine metropolitan areas with populations of five million or more.

California led the nation with the highest percentage of foreign-born residents with 25.9 percent, followed by New York State with 19.6. The metropolitan areas with the largest percentage of foreign-born populations were Los Angeles were 29.6 percent and New York with 22.8.

In New York City, said John H. Mollenkopf, a demographer at the City University of New York, 43 percent of the residents are foreign born and another 9.2 percent are the children of two foreign-born parents. In 1900, Professor Mollenkopf said, only 23 percent of New Yorkers had native-born parents.

The Census Bureau's report put at 55.9 million the number of people of so-called foreign stock, which includes 28.4 million foreign born, 14.8 million native born with two foreign- born parents, and 12.7 million of mixed parentage. That group is likely to grow, in part because the proportion of births to foreign-born women rose to 20.2 percent in 1999 from 6 percent of all births in 1970, the report said.

While the median ages of the foreign-born and native populations barely differed, foreign-born residents fell disproportionately between the ages of 25 and 54. The percentage of foreign-born residents in that age group was 58.7, compared with 41.7 percent of native Americans.