"The number is still around 650,000," said Dr. Web.

On April 16, the company continued, it said 595,000 different Macs were registered on the botnet, while the next day, April 17, the count was over 582,000.

Symantec's O Murchu said Dr. Web is right.

"We've been talking with them about the discrepancies in our numbers and theirs," said O Murchu in an interview Friday. "We now believe that their analysis is accurate, and that it explains the discrepancies."

When asked for comment, Kaspersky Lab said it was looking into the matter.