The highway that rings Washington, D.C., is the Capital Beltway in more ways than one.

Not only does that crowded highway encircle the capital city. It also is home to some of the nation's most successful capitalists, according to the Census Bureau.Of the nation's 10 highest-income communities, fully half are suburbs of Washington.

What's more, if the District of Columbia were a state it would rank third in per capita income among the states, the bureau said Wednesday in releasing final per person income figures for 1987.

While the folks with the least cash live in Starr County, Texas, residents of Falls Church, Alexandria and Arlington, Va. - all in suburban Washington - rank first, second and third, respectively.

On a state-by-state basis, residents of Connecticut had the highest average incomes, while those in Mississippi brought home the smallest paychecks, according to a new Census study of per capita income as of 1987.

The report calculated average income for all 50 states and included the 10 richest and 10 poorest counties.

Among counties, the top per capita income was $23,169 in Falls Church, Va., followed by $22,200 in Alexandria and $22,181 in Arlington.

While the Census Bureau calculates income based on counties, the cities of Falls Church and Alexandria were included because they are independent of any county.

Marin County, Calif., an affluent San Francisco suburb, finished fourth on the income list at $21,611 per person.

Then it was back to the suburbs of Washington for fifth-ranking Montgomery County, Md., at $21,576.

New York County, N.Y., better known as Manhattan, ranked sixth with an income of $20,904 per person. Next was the fifth Washington suburb to make the top 10, Fairfax County, Va., with a per capita income of $20,805.

Rounding out the top income areas were Nantucket, Mass., $20,405; Somerset, N.J., $20,171, and Fairfield, Conn., $20,093. The last two are distant suburbs of New York.

At the other end of the scale is Starr County, on the Mexican border north of McAllen, and with about 36,000 residents. Those folks averaged just $3,464 each.

Next up the list of the 10 lowest-income counties were two in South Dakota. Residents of Shannon County, south of Rapid City on the Nebraska border, averaged $3,943. Income was $4,250 per capita in Buffalo County, southeast of Pierre.

Maverick County, Texas, another Mexican-border spot northwest of Starr County, was fourth lowest in income at $4,269 per person.

Next were two Mississippi counties. Jefferson, in the southwestern part of the state, had per capita income of $4,413, while Tunica, south of Memphis, Tenn., averaged $4,625.

Next, with per capita income of $4,646, was Zavala County, Texas, adjacent to Maverick County.

Zieback County, in north central South Dakota, was that state's third entry among the 10 poorest in the country. Residents averaged $4,688 each.

Rounding out the bottom 10 were Wade Hampton borough in western Alaska at $4,727 per person and Owsley County, in the hills of eastern Kentucky, at $4,747.

The Census study ranked only the 10 counties with the highest and lowest incomes, the states and the District of Columbia. Detailed figures on the rest of the nation's counties will be published later, the agency said.

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(Additional information)

Income rundown

Connecticut $16,094

New Jersey $15,028

Dist. of Col. $14,778

Maryland $14,697

Massachusetts $14,389

Virginia $13,658

New Hampshire $13,529

Alaska $13,263

California $13,197

New York $13,167

Delaware $12,785

Nevada $12,603

Florida $12,456

Illinois $12,437

Rhode Island $12,351

Hawaii $12,290

Minnesota $12,281

Colorado $12,271

Washington $12,184

Michigan $11,973

Pennsylvania $11,544

Arizona $11,521

Kansas $11,520

Wisconsin $11,417

Georgia $11,406

Ohio $11,323

Vermont $11,234

Missouri $11,203

Iowa $11,198

Nebraska $11,139

Indiana $11,078

Oregon $11,045

North Carolina $10,856

Texas $10,645

Maine $10,478

Tennessee $10,448

South Carolina $9,967

Oklahoma $9,927

Wyoming $9,826

North Dakota $9,641

Alabama $9,615

New Mexico $9,434

Kentucky $9,380

Montana $9,322

Utah $9,288

Idaho $9,159

Arkansas $9,061

West Virginia $8,980

Louisiana $8,961

South Dakota $8,910

Mississippi $8,088