Sophos said it identified 1,157 new viruses in July, the highest number since December 2001.
"December 2001 still holds the title for having the most reported virus threats with 1,256, but without a doubt, July's volume is extraordinary and concerning," Chris Kraft, senior security analyst at the antivirus vendor, said in a prepared statement.
Sophos noted the trend in its top ten virus chart for July, released Tuesday. The company has been publishing top ten virus charts since August 1999.
The Zafi worm dominated the virus chart for July, accounting for more than half of all viruses reported to Sophos.
"The Zafi worm, which sends itself via email using a variety of languages is likely to linger and continue to wreak havoc on companies' defense systems," Kraft said.