Publicis Groupe bolstered its already substantial investment in Internet advertising last week, acquiring Rosetta Marketing Group for $575 million in cash along with deferred payments based on the agency’s performance.

Publicis’ acquisition of Rosetta —declared the second largest digital marketing agency in the U.S. by Advertising Age in 2010— shows that the French group has moved more aggressively than its rivals to increase its presence in digital interactive advertising. The Paris-based advertising colossus has made other U.S. digital agency purchases over recent years, acquiring Digitas for $1.3 billion in 2006 and Razorfish for $530 million in 2009.

New Jersey-based Rosetta Marketing Group specializes in areas like search engine advertising and direct marketing, and also has a strong position in the health care business, an area of focus for Publicis. Rosetta’s clients include Bristol Myers Squibb; Hewlett-Packard; Johnson & Johnson; Research In Motion, the developer of the BlackBerry; and the T-Mobile division of Deutsche Telekom.

The 13-year-old agency will remain an autonomous operating unit within Publicis. Chris Kuenne, founder & CEO of Rosetta will continue to lead and will be reporting to Jean-Yves Naouri, Publicis’ Chief Operating Officer.