This paper reports on how some features of the Macintosh operating system, versions 7.1 through 8.1 and perhaps later versions, could be used to implement a virus attack unlike any seen previously on the Macintosh, but which bears some resemblance to a “Companion Virus” style of attack as seen under MS-DOS. We briefly discuss some methods used in the implementation of companion viruses under MS- DOS, and also examine techniques used by other Macintosh viruses. Following an examination of the details of our attack, we discuss generic countermeasures to a virus using the attack, one of which in particular appears very effective against the attack.