ComScore and Nielsen are locked in a legal fight over intellectual property rights.
In a suit filed last week in Virginia Eastern District Court, Nielsen claims that its rival online measurement service is violating its patents.
The company is seeking unspecified damages and claiming that comScore has been copying the way it tracks web traffic.
At stake is a highly lucrative business, as scores Internet companies receive from the tracking companies help them set ad rates.
"While we support vigorous competition in the marketplace, that competition must be fair and respect intellectual property rights. Nielsen does not take such matters lightly and only after thorough analysis and consideration did we file this action. We look forward to a fair and appropriate resolution," Nielsen said in a statement.
ComScore has countersued in United States District Court, claiming that Nielsen infringed on its patents, according to Reuters.
A comScore spokesperson did not immediately respond to TheWrap's request for comment.
News of the Nielsen suit first broke on the Hollywood Reporter.