Peer-to-peer (P2P) networks continue to be popular means of trad- ing content. However, very little protection is in place to make sure that the les exchanged in these networks are not malicious, mak- ing them an ideal medium for spreading malware. We instrument two different open source P2P networks, Limewire and OpenFT, to examine the prevalence of malware in P2P networks. Our results from over a month of data show that 68% of all downloadable re- sponses in Limewire containing archives and executables contain malware. The corresponding number for OpenFT is 3%. Also, most infections are from a very small number of distinct malware. In particular, in Limewire, the top three most prevalent malware account for 99% of all the malicious responses. The corresponding number for OpenFT is 75%. We also investigate the sources of ma- licious responses. To our surprise, 28% of all malicious responses in Limewire come from private address ranges. In OpenFT, the top virus, which accounts of 67% of all the malicious responses, is served by a single host. Further, our study provides a useful in- sight into ltering malware: ltering downloads based on the most commonly seen sizes of the most popular malware could block a large portion of malicious les with a very low rate of false posi- tives. While current Limewire mechanisms detect only about 6% of malware containing responses, our size based ltering would detect over 99% of them.