The popular file-sharing service Kazaa is being sued by a Romanian national who claims to have written the source code for the program.
Kazaa is popular with millions of surfers
Kazaa Media Desktop enables computer users to connect to a so-called peer-to-peer network online.
Fabian Toader claims to have written the computer code in 2000 while working as a freelancer for Kazaa.
He is seeking $25 million in compensation from Sharman Networks which bought the rights to the code.
Twist to dispute
Mr Toader worked in Romania at the time and has since moved to the US where he is a programmer with Microsoft.
The legal battle is a twist for a company, more used to litigation from Hollywood studios and recording companies over the copyright of the files swapped on its network.
"Sharman has made millions using my software. I just want to be fairly compensated for my contribution," Mr Toader said in a statement.
The lawsuit, filed at the beginning of March, is part of a long line of litigation between Sharman and Mr Toader.
In the summer the company sued Mr Toader in the Washington State Superior Court, alleging he tried to blackmail the company.
This is the second time Mr Toader has counter-sued.
"Sharman regards this recently filed Los Angeles law suit as nothing more than Mr Toader's more recent shakedown effort," a Sharman spokesman said in a statement.
The company claims that the work Mr Toader did on the software program was on a work for hire agreement which expressly stated that Kazaa owned all the rights.
Mr Toader claims he never signed a contract with Kazaa and, under US copyright laws, he is therefore the owner of the program.
Battle with copyright holders
Sharman is no stranger to lawsuits as Kazaa continues to court controversy.
Moves are afoot in the US legislature to force peer-to-peer networks to have much stricter limits on their services.
In February Sharman's Sydney headquarters were raided by investigators acting on behalf of the Australian recording industry.
They were looking for evdience to support a copyright infringement case.
The latest version of Kazaa has been downloaded more than 335 million times.
The popularity of peer-to-peer networks has alarmed Hollywood film studios, recording companies and other copyright holders, all of which claim to have lost millions as a result of file-sharing.