The number of people using several music-swapping sites dropped the week after the music industry threatened to sue them.
The RIAA says swapping and copying songs will
Kazaa and Morpheus - two of the most popular file-swapping services - said they had 15% fewer users the week beginning 6 July.
This was the week after the Recording Industry Association of America (RIAA) threatened to sue individual computer owners who consistently swapped large amounts of songs over the internet.
The decrease means as many as one million fewer people using the Kazaa network, according to Neilsen Netratings.
"With the negative publicity and threat of steep fines, some surfers appear to be backing off," said Greg Bloom, a senior internet analyst with Neilsen Net ratings.
"However, with millions of loyal users, these applications aren't likely to go off-line in the near future."
Sharman Networks, which runs Kazaa, said fluctuations in user numbers were normal.
"Aside from normal seasonal drops experienced on the internet, we regularly see fluctuations in the number of users online, and there are numerous factors that play a part in these cyclical changes, but they are rarely permanent," the company said.
The RIAA took action against individuals after it failed in an attempt to force Morpheus and another network, Grokster, to stop their services. A judge ruled they were not responsible for what was traded over the networks.
But the RIAA then won a court case forcing telecommunications giant Verizon to hand over the identities of large-scale file-swappers.
The RIAA then threatened $150,000 (£90,000) suits against users, which it plans to instigate in August.
In June, the RIAA's president Cary Sherman said if people did not want to be sued, they should block access to their hard-drives from other computer users, or uninstall file-sharing software.