The gap between rich and poor is as great in the District as in any other major city and has grown more here than in most places, a widening chasm that troubles government leaders.

A study to be released today by the D.C. Fiscal Policy Institute said the top 20 percent of the city's households have 31 times the average income of the 20 percent at the bottom. The gap in the District is fed by extremes at both ends: The poor have less average income than in most of the country's 40 biggest cities, and the rich have more.

The persistent gap between rich and poor has been fueling debate over whether the national economic recovery is helping all Americans. The study deepens the picture of an increasingly fractured city, where poverty and wealth both grew in the last decade. The average household income for the top group was $186,830, and the average income for the poorest group was $6,126.

"The rich got richer and the poor didn't get richer," said Stephen Fuller, a regional economist at George Mason University in Fairfax. "The poor can't afford to get out of Washington to the suburbs. . . . Our wealthy class got wealthier in the 1990s, and it didn't trickle down to the bottom."

The new report identifies the District, Atlanta and Miami as the big U.S. cities with the largest income gaps.

Another recent analysis, by the Lewis Mumford Center at the State University of New York at Albany, found that the District now ranks higher among economically polarized cities than it did in 1990. The analysis, by Brian Stults, a sociology professor at the University of Florida, employed a standard technique to analyze income inequality and ranked the District among the five big cities with the largest gap between rich and poor.

The D.C. Fiscal Policy Institute study measured 1999 income, but a co-author, Ed Lazere, said the income gap is not likely to have closed since then. Nationally, the gap between rich and poor widened from the 1970s until the early 1990s, and has inched up slightly since.

The trend, experts say, reflects a growing gap in wages between skilled, educated workers and those with no skills, as well as social changes such as a growing number of single parents, who have lower incomes than married couples. Although some gap is expected, they see the trend as a disturbing reflection of an economy in which people without college educations will be stuck at the bottom.

The city's richest and poorest households could not be more different, according to Lazere's analysis. Half of the richest households, with incomes starting at $89,814, are married. Among the poorest, where incomes topped out at $14,000, six in 10 were single, living alone. Single mothers accounted for less than 10 percent of the richest households, and more than a quarter of the poorest ones. Nearly all the working-age adults held jobs in the richest households, but only about half did in the poorest ones.

Using numbers from another census survey, Lazere's study calculated that the incomes of the city's richest households rose 38 percent over the decade, while those of the poorest went up 3 percent.

Tony Bullock, a spokesman for Mayor Anthony A. Williams (D), said the gap is the product of complex forces, including poor city services and poor schooling, that have persisted for decades and cannot be fixed overnight.

"We have a large concentration of poverty where no matter what we seem to do to bring investment into the District, a certain population is not able to access the kind of employment opportunities that come from a growing tax base," he said. "But it is our hope that we can improve in the future."

Bullock said the attractiveness of the city to high-income households is good for its tax base, and the study agreed. It said high-income families in the Washington region are more likely to live in the city than are affluent families in most other big metro areas.

Those at the top benefit from the District's unique job bank of high-paid employment related to the federal government, including lobbying and contracting. A single young professional can earn $100,000 in his or her first year out of law school.

At the other end of the income scale, Lazere's study said, the D.C. minimum wage, $6.15 an hour, is worth less when inflation is taken into account than it was worth in 1979. The purchasing power of the city's maximum welfare benefit -- $379 for a family of three -- fell by nearly a third over the decade, it said.

A bill co-sponsored by D.C. Council members David A. Catania (R-At Large) and Sandy Allen (D-Ward 8) would raise the D.C. minimum wage to $6.60 an hour next year and to $7 an hour by January 2006. It would be the first increase since 1997 in the D.C. minimum wage, which is set at $1 above the federal level. Catania said yesterday that he is confident that it will pass, and that he also wants the city to beef up its training programs for less-skilled workers.

"I don't want to focus so much on income disparity," he said. "The government should focus more on how to lift these workers out of poverty and help them make better wages."

Lazere said he is concerned that the mayor's efforts to boost the city's population by 100,000 over the next decade and attract high-income residents could squeeze out the poor through gentrification if the city does not expand its assistance to low-income workers.

"At the high end, the city already is attractive," he said. "Specific policies to attract more high-income families may not be needed and may exacerbate the problems for our neediest residents."