Six BitTorrent sites hosting links to others with illegal copies of TV shows have been targeted in lawsuits by the Motion Picture Association of America.
It is a shift in focus for the MPAA. Since it started legal action against file-sharers in December, its targets have been film indexing sites.
A recent survey said TV programme downloads had risen by 150% in a year.
About 70% were using BitTorrent sites, according to the Envisional research. Of the total downloaders, 18% were within the UK, it said.
In March, TV downloading made headlines with the appearance of the long-awaited new series of Doctor Who on the net before it was even broadcast.
The MPAA said it was worrying.
"There are thousands of people in the entertainment industry who are working to develop, produce, and promote television shows. Those shows and those jobs are worth protecting," said Dan Glickman, MPAA chief.
"Every television series depends on other markets-syndication - international sales - to earn back the enormous investment required to produce the comedies and dramas we all enjoy and those markets are substantially hurt when that content is stolen."
The percent of working servers has dropped by more than 40% since it started action, said the MPAA.
"Since we began shutting these sites down, the time that it takes to download a file on BitTorrent has increased exponentially which means the experience of downloading copyrighted films and TV shows is not what it used to be," said Mr Glickman.
"We intend to make it even worse. Protecting the television industry is essential."
With BitTorrent software, server sites do not host the files being shared. Instead, they host links, called "trackers" which tell people where to go to get the files.
More than 90% of the sites that the MPAA has sued so far have been shut down entirely.
The sites which have been closed, such as LokiTorrent, UK Torrent and s0nicfreak, now carry warning messages from the MPAA that read: "You Can Click But You Cannot Hide."
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