Septemer 8, 2005

FOR IMMEDIATE RELEASE

Dr. Susan Froshauer, President and Chief Executive Officer of Rib-X Pharmaceuticals, will present at the Thomas Weisel Partners Healthcare Tailwinds 2005 Conference

Thursday
September 8, 2005
3:15pm
Boston

Rib-X Pharmaceuticals is a product driven small molecule drug discovery and development company focused on the structure-based design of new classes of antibiotics. The Company's drug discovery strategy exploits its proprietary high-resolution crystal structure of the 50S subunit of the ribosome, which performs an essential role in the fundamental process of protein synthesis. The 50S is the target for many clinically valuable classes of antibiotics, including those used to treat infections caused by serious, drug-resistant, community-acquired and hospital pathogens. The Company's integrated research approach combines state of the art, computational analysis, x-ray crystallography, medicinal chemistry, microbiology and biochemistry, allowing for the rapid synthesis of new classes of antibiotics, which avoid typical antibiotic resistance mechanisms. During 2005, Rib-X expects to initiate Phase I studies for at least one clinical candidate from its most advanced program. This program (the RƒÓ-01 program) has yielded attractive, orally active compounds with attributes appropriate for therapeutic applications in either the hospital Gram-positive or community respiratory tract infection marketplace. Rib-X's iterative approach has yielded several distinctive chemical classes in addition to those defined by the RƒÓ-01 program. The Company is currently well-positioned to develop a pipeline of differentiable products that are expected to address distinct markets for the treatment of bacterial infections. For more information on the Company, the ribosome and the Rib-X mission, please visit the Company website at www.rib-x.com.

For Additional Information:
Rx Communications Group
Rhonda Chiger
917-322-2569

About Rib-X
Rib-X Pharmaceuticals is a small molecule drug discovery company focused on the structure-based design of new classes of anti-infective agents. The Company’s drug discovery strategy is focused on the exploitation of its proprietary high resolution crystal structure of the 50S subunit of the ribosome to which many known antibiotics bind. The Company’s integrated research approach combines state of the art, proprietary computational analysis, x-ray crystallography, medicinal chemistry, microbiology and biochemistry, allowing for the rapid synthesis of new agents which avoid typical antibiotic resistance mechanisms. Utilizing these tools, Rib-X is optimizing leads to create new classes of antibiotics.

Contacts
Rx Communications Group
Rhonda Chiger (investors)
917-322-2569
rchiger@rxir.com