HEMPSTEAD, N.Y., June 4— This village and Garden City have been municipal neighbors for decades, one stop away from each other on the Long Island Rail Road, a four-minute ride. But despite their shared boundary, they are worlds apart.
Garden City is home to many executives. It has a median family income of $120,305, houses that look like pages out of decorator magazines, downtown businesses like Saks and Fidelity Investments and a population that is 92 percent white. The mayor and village and school board members are all white.
Hempstead is a working-class community. It has a $46,675 median family income, midrise apartment buildings, generic middle-class homes, a downtown with storefront churches, thrift shops and Spanish restaurants, and a population that is 51 percent black and 32 percent Hispanic. The mayor is black, as are all but one of the village and school board members.
Such separation of blacks and whites is the rule, not the exception, across Nassau and Suffolk Counties, so much so that a new study comparing rates of integration in different communities ranks Long Island as the nation's most segregated suburb.
''African-Americans have faced isolation far more than any other group, especially on Long Island,'' said David Rusk, an international consultant on urban and suburban issues who analyzed the segregation patterns. Other minority groups are more integrated, the figures show.
Erase Racism, a year-old nonprofit group sponsored by the Long Island Community Foundation in Jericho, hired Mr. Rusk to research the study and present it at a conference in Islandia on Wednesday. What he finds is a new version of an old problem.
The central finding is that 74 percent of Long Island's blacks would have to move to be evenly dispersed across the population. Mr. Rusk drew his conclusions from an analysis of the 2000 federal census by the Lewis Mumford Center for Comparative Urban and Regional Research at the State University of New York at Albany.
Segregation is far more extreme on Long Island, for example, than in two major Washington suburbs. In Montgomery County, Md., 41 percent of blacks would have to move to be evenly distributed. In Fairfax County, Va., the figure would be 38 percent. The nation's 100 largest metropolitan areas average 60 percent. The only places more segregated than Long Island are big cities like Detroit at 85 percent, Milwaukee at 82 percent and Chicago at 81 percent.
''It's almost like a township in the South African sense'' during apartheid, said another expert, Andrew Beveridge, a Queens College sociologist who has made similar findings.
Why Long Island is segregated is a matter of debate. Mr. Rusk blamed the tax, zoning, housing and education policies of Long Island's ''frustrating maze of little-box governments: 109 villages, towns and cities and 129 school districts.''
''The unspoken agenda of most little-box councils, or most little-box school boards, is 'to keep our town, or our schools, just the way they are for people just like us,' whoever us happens to be,'' said Mr. Rusk, a former mayor of Albuquerque. ''The result is that Long Island is highly divided by race and ethnicity. For black residents, Long Island is the most segregated suburb in America.''
Segregated patterns are continued in various ways, said V. Elaine Gross, director of Erase Racism, a group of business and community leaders that tries to foster integration on Long Island. ''Testers show that we still have racial steering and blockbusting going on,'' she said. Blacks often have a hard time getting mortgages, even with good credit histories, and are victimized by exorbitant charges, she said.
Long Island's suburbs got off to a segregated start when discrimination was openly accepted. The initial leases in Levittown, America's pioneering post-World War II suburb, announced in bold capital letters that its homes were not to be ''used or occupied by any person other than members of the Caucasian race.''
Although the United States Supreme Court nullified such rules in 1948, the Levitt company continued to discriminate. Today blacks make up less than 1 percent of that community's population.
Fairfax and Montgomery Counties are models that thrive on a formula of centralized schools and strong county governments that reserve 15 percent of new housing for low- and moderate-income families, Mr. Rusk said. But he doubts that Balkanized Long Island would seek that solution. ''They won't do it voluntarily,'' he said. ''To overcome the nimbyism, someone's got to tell them to do it,'' through a court order or state laws.
The Long Island Regional Planning Board's director, Lee E. Koppelman, lamented that since he identified 13 minority communities in the 1960 census, ''I really don't see any improvement'' in housing diversity. He added: ''Exclusionary zoning, unfortunately, is a reality. You can have all the laws on the books, but if you don't have the will of the people, it won't work.''
Photos: Garden City, above, is home to many executives and has a population 92 percent white. The mayor and the school board members are all white. In Hempstead, below, the population is 51 percent black and 32 percent Hispanic. The mayor is black, as are all but one of the village and school board members. (Photographs by G. Paul Burnett/The New York Times)