CHICAGO – October 3, 2005 – The Archipelago ExchangeSM (ArcaEx®), the nation's first totally open, all-electronic stock exchange, today announced that Avalon Pharmaceuticals a company focused on the discovery and development of small molecule therapeutics (NASDAQ : AVRX) has dually listed on ArcaEx.

"We are excited to welcome Avalon Pharmaceuticals to the Archipelago Exchange," said Sam Long, Managing Director of ArcaEx's Corporate Client Group. "We look forward to seeing other innovative and pioneering companies listing on ArcaEx as we make the listings business more competitive through our new service and new data products."

In addition to providing the advantages of fully-electronic trading, ArcaEx enables NYSE- , Amex- and Nasdaq-listed companies and their investors to reap the benefits of ArcaEx's investor focused market structure. ArcaEx operates as an all-electronic auction market, with an open limit order book matching buyers and sellers directly based on simple price and time priority, leveling the playing field for all market participants.

Currently over 230 companies are listed for trading on ArcaEx. In addition to its own listings, the exchange trades all NYSE, Nasdaq, and Amex listed stocks.

About ArcaEx

Archipelago Holdings (PCX: AX) operates The Archipelago Exchange SM (ArcaEx®) the first totally open all-electronic stock exchange in the United States. ArcaEx customers can trade over 8,000 equity securities, including securities listed on the New York Stock Exchange ®, Nasdaq®, American Stock Exchange® and ArcaEx®. In addition to offering core execution services, ArcaEx provides corporate clients with listing services and innovative data products. In 2004, Archipelago launched ArcaVision to provide corporate and trading clients with more transparent market data, analytical tools and access to market-wide and ArcaEx specific trading information. Archipelago Holdings completed its initial public offering in August 2004. For more information please visit http://www.arcaex.com.

About Avalon

Avalon Pharmaceuticals is a biopharmaceutical company focused on the discovery and development of small molecule therapeutics for the treatment of cancer. Avalon seeks to discover and develop novel therapeutics through the use of a comprehensive, innovative and proprietary suite of technologies based upon large-scale gene expression analysis which it calls AvalonRx® Avalon Pharmaceuticals was established in 1999 and is headquartered in Germantown, Maryland.

For more information please visit www.avalonrx.com.

Forward-Looking Statements

Certain statements in this press release may contain forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. These statements are based on Archipelago's current expectations and involve risks and uncertainties that could cause Archipelago's actual results to differ materially from those set forth in the statements. There can be no assurance that such expectations will prove to be correct. Factors that could cause Archipelago's results to differ materially from current expectations include: general economic and business conditions, industry trends, competitive conditions, regulatory developments as well as other risks or factors identified in the Company's filings with the Securities Exchange Commission, including its Report on Form 10-K for the fiscal year ending December 31, 2004 which is available on the Company's website at http://www.archipelago.com. You should not place undue reliance on forward-looking statements, which speak only as of the date of this press release. Except for any obligation to disclose material information under the Federal securities laws, Archipelago undertakes no obligation to release publicly any revisions to any forward-looking statements to reflect events or circumstances after the date of this press release.

Contact:

Archipelago
Margaret Nagle
312-442-7083