I've fished around the Yellowstone area quite a bit, and have been very successful, but have never gone with a guide service. Really, you don't need one to be succesful on the Yellowstone. Just ask around and people are willing to lead you in the right direction. My favorite shops are Dan Baileys and George Andersons Yellowstone Angler in Livingston, and Sweetcast Angler in Big Timber (east of Livingston on I-90). Everybody in there is helpful and will lead you on the right track as far as floats, fly selections, and shuttle services, etc. Some things to consider when fishing the Yellowstone:
- If there's any rain in the park at all ,the river turns to dark chocolate milk in a matter of hours(you can see it happen while floating). However, there are rivers around there, like the Boulder, that offer excellent fishing when the 'stone is messed up. Also, there's the famous Pay-to-play spring creeks running through Paradise Valley that can have good fishing for trophy fish, but they cost $100 per rod.
- It's a big, fast river with some dangerous area's. Personally, I've had 2 near death experiences on that river, but thats probably just my bad luck

. The fish are large aswell, heavier tippets are a good idea.
- In my opinion, the sections below Livingston are the best, there's less of the Yellowstone Cutties, but there are massive Browns and Rainbows. Plus, there's far less people on that section of the river, for a more wilderness experience. I floated from Springdale to Grey Bear and had alot of success with big hopper/attractor patterns. Steamers in Yellow/Brown/Olive are good to.