Sonia Kuester Pfaffenroth focuses her practice on helping clients address complex antitrust issues in the US and globally. She rejoined the firm in 2017 from the Antitrust Division of the US Department of Justice (DOJ) where she served most recently as Deputy Assistant Attorney General for Civil and Criminal Operations. In that role, Ms. Pfaffenroth 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, Ms. Pfaffenroth 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, Ms. Pfaffenroth 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.

Ms. Pfaffenroth originally joined Arnold & Porter in 2005 after clerking for Judge Paul Friedman of the US District Court for the District of Columbia.


  • 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 US 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.


Antitrust Agency Insights: Developments at the US Antitrust Enforcement Agencies—May 2019
Antitrust/Competition Newsletter
DOJ Changing Its Antitrust Approach to FRAND and SEPs (pdf)
Intellectual Property & Technology Law Journal, Volume 31, No. 4
Reforming Section 2 Law and Enforcement
Competition Roundtable: Challenging Monopolies in Court – Where Have We Been and Where Are We Going?, American Antitrust Institute, Washington, DC
Navigating Government Shutdowns: How to Advise Clients and Other Tips
American Bar Association Webinar
US Antitrust Year in Review: The Trump Administration's Second Year
Arnold & Porter Webinar


The Legal 500 US
Antitrust: Cartel (2018-2019)
Antitrust: Civil Litigation/Class Actions (2018-2019)
Antitrust: Merger Control (2019)
"40 in their 40s" – Notable Women Competition Professionals in Americas (2019)


  • JD, Stanford Law School
  • BA, Carleton College
  • District of Columbia
  • California
Government & Military Service
  • Deputy Assistant Attorney General for Civil and Criminal Operations, Antitrust Division, US Department of Justice
  • Deputy Assistant Attorney General for Civil Enforcement, Antitrust Division, US Department of Justice

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