Mike, the quick answer is that if you're not a Yukon resident and a Canadian citizen, you'll need to hire a guide. But here are the details from the folks who make the rules:
http://www.environmentyukon.gov.yk.c...unt2008-09.php. That link takes you to info about the bison hunts, but you'll see links to other big game hunting info as well. For anyone interested in small game-- i.e., the stuff that flies, you don't need to hire a guide. Hope that helps.