When the forces of the music industry go after websites for using copyrighted content without permission, that content tends to come in the form of MP3 files or YouTube videos.

But the National Music Publishers’ Association, a trade group representing thousands of publishers, noted in an online news conference on Monday that the Internet was also filled with sites that reprint song lyrics without licenses, selling advertising based on the enormous traffic they attract. According to the association, there are five million Google searches each day for lyrics, and more than half of all lyric page views are on unlicensed sites.

David Israelite, the president of the trade group, said that his organization was filing take-down notices against what it called the 50 “worst offenders” based on a web search conducted by David Lowery, a researcher at the University of Georgia. Mr. Lowery, best known as the lead singer of the alternative rock bands Camper Van Beethoven and Cracker, has become an outspoken advocate for artists’ rights in the digital age, which has often put him at odds with technology companies large and small.

“These lyric sites have ignored the law and profited off the songwriters’ creative works, and N.M.P.A. will not allow this to continue,” Mr. Israelite said in a statement, referring to his organization. “This is not a campaign against personal blogs, fan sites or the many websites that provide lyrics legally. N.M.P.A. is targeting 50 sites that engage in blatant illegal behavior, which significantly impacts songwriters’ ability to make a living.”