In the running cross-border rivalry between Maryland and Virginia, Fortune magazine has just added another 500 points to the Old Dominion’s score.

These Enhanced Night Vision goggles are made by ITT Exelis. The Virginia based company just joined the Fortune 500 list. (Jeffre MacMillan)

This year’s list puts Exxon Mobil Corp. at the top — which, the county’s economic development authority notes, keeps its refining, retailing and marketing headquarters in Fairfax County. Wal-Mart dropped to second place.

The county’s economic development authority said that Fairfax County now has more firms in the Fortune 500 than 34 other states. Alan Fogg, a spokesman for the economic development authority, said Maryland has six Fortune 500 companies, while the District has three.

Competition between Maryland and Virginia for the claim of best business environment has, if anything, only grown in proportion to the political ambitions of Govs. Martin O’Malley (D) and Robert F. McDonnell (R). Both states also made highly public pitches for Bechtel — as did the District — before the company decided to move to Fairfax.

The 2012 Fortune 500 — which is based on annual 2011 revenues — appears in the magazine’s May 21 edition and includes the following Fairfax County companies:

Freddie Mac, mortgage securities (No. 25)

General Dynamics, defense and aviation (No.92)

Northrop Grumman, defense and security (No. 104)

Capital One, financial services (No. 148)

CSC, technology consulting and systems integration (No. 162)

SAIC, engineering and research (No. 245)

NII Holdings, mobile communications services (No. 369)

ITT Exelis, defense and security (No. 422)

Booz Allen Holding, technology consulting (No. 439)

Gannett, publishing and television (No. 465)

“Thirty years ago no Fortune 500 companies were based here, but now 2 percent of the largest companies in the nation are headquartered in one place — Fairfax County,” Gerald L. Gordon, the economic development authority’s president and chief executive, said in a written statement.

The economic authority also made several interesting observations, such as:

— “The Fairfax County total accounts for 50 percent of the 20 listed companies from the Washington, D.C., region.

— “The Fairfax County companies account for 41.7 percent of the 24 companies from Virginia on this year’s list.

— “The Fairfax County companies had revenues of nearly $218 billion in 2011, which was 61.3 percent of the revenue of all Fortune 500 companies headquartered in Virginia.

— “Fairfax County’s Fortune 500 companies accounted for 45.3 percent of the revenues of all Fortune 500 companies based in the Washington region.

— “The Fairfax County companies had more than twice the combined revenue of the Fortune 500 companies based in Maryland -$218 billion compared to $95 billion.

— “The Fairfax County companies had 36 percent more combined revenue than the Fortune 500 companies based in Washington, D.C. — $218 billion compared to $160 billion.

— “The revenue of the Fairfax County companies roughly equaled the gross domestic product of Ireland, according to the International Monetary Fund.”