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Sonia Kuester Pfaffenroth focuses her practice on helping clients address complex antitrust issues in the U.S. and globally. She rejoined the firm in 2017 from the Antitrust Division of the U.S. Department of Justice (DOJ) where she served most recently as Deputy Assistant Attorney General for Civil and Criminal Operations. In that role, Sonia was responsible for supervising both civil and criminal antitrust enforcement efforts, as well as the Division's work with antitrust and competition law enforcement agencies worldwide.

While at the DOJ, Sonia oversaw a number of the Division's most significant matters, including the Division's challenges to the Anthem/Cigna and Aetna/Humana mergers and the American Express litigation. In addition to her responsibilities with respect to the Division's international program, she also supervised a broad range of legal policy initiatives and competition advocacy efforts. She previously served as Deputy Assistant Attorney General for Civil Enforcement at the Antitrust Division and Chief of Staff and Senior Counsel to former Assistant Attorney General for Antitrust Bill Baer.

Before leaving to join the DOJ in 2013, Sonia was a partner at the firm, where she represented companies in a range of industries in international cartel investigations, merger and acquisition reviews and civil antitrust litigation, as well as providing day-to-day antitrust counseling.

Sonia originally joined Arnold & Porter in 2005 after clerking for Judge Paul Friedman of the U.S. District Court for the District of Columbia.


  • Mallinckrodt Pharmaceuticals, in federal and state class action and non-class action litigation alleging antitrust and marketing related claims regarding one of its leading products. Obtained complete defense verdict in trial in bankruptcy proceeding, defeating certain antitrust and civil RICO claims.
  • Bayer in federal litigation alleging claims of monopolization, tying, and exclusive dealing involving companion animal flea and tick products.
  • Fuel company in DOJ criminal and civil bid rigging and False Claims Act investigations.
  • Koito Manufacturing Co., Ltd. in DOJ criminal investigation regarding automotive lighting systems and in related civil litigation.
  • Fujikura Ltd. & Fujikura America Inc. in direct and indirect purchaser class actions alleging price-fixing regarding automobile wire harnesses and related products.
  • Financial division of a major multinational corporation in connection with criminal and regulatory investigation of the municipal bond reinvestment markets by the DOJ and certain state attorneys general, as well as in related multidistrict litigation.
  • Laboratoires Fournier in an antitrust jury trial in the U.S. District Court for the District of Delaware involving allegations of predatory innovation. Case settled just prior to jury deliberations.
  • Zimmer Holdings in DOJ criminal price-fixing investigation involving orthopedic implant devices and in related class action litigation.


Chambers USA
Antitrust — Washington, D.C. (2024)
The Legal 500 US
Antitrust: Cartel (2018-2019, 2022)
Antitrust: Civil Litigation/Class Actions (2018-2019, 2022)
Antitrust: Merger Control (2019)
Global Competition Review
"Women in Antitrust" (2021)



  • J.D., Stanford Law School, Order of the Coif
  • B.A., Carleton College, magna cum laude


  • District of Columbia
  • California

Government & Military Service

  • Deputy Assistant Attorney General for Civil and Criminal Operations, Antitrust Division, U.S. Department of Justice
  • Deputy Assistant Attorney General for Civil Enforcement, Antitrust Division, U.S. Department of Justice