Load it into a dynamic text object, obviously, but then enable that object for html text, once this is done you can modify the contents of your external text file allowing you to use html tags such as <i>itallic text</i> when it is loaded into the mc.

Also I recently found that dynamic text does not work terribly well beneath a lot of masks, or any for that matter, no doubt flash 8 quirk, so you might want to check that out also.