The tackle and gear you need for fly fishing in Yellowstone National Park is really not much different that what you would use fishing many other small to medium trout streams across the nation. A nine foot, 5 weight fly rod would work for any of the streams. A four weight is okay but would be a light side. A 6 weight would work but I prefer the 5 weight over the 6 weight when it comes to presenting dry flies. The fly line should be a weight forward floating line. The fly reel should match the rod, have a good drag, and be as light as possible. A vest, waist pack or sling pack would work well to store your on the water needed items and just a matter of your personal choice. Waders and wading boots are necessary. You do not want to wet wade the streams in the park, especially if you notice there are lots of leeches in the water. You may want to have several other accessory items that would be no different than what you would like to have on any other trout stream. A landing net is an optional item, for example. You should have 9 ft. leaders ranging from 3x to 6x. 12 foot leaders are rarely needed. Shorter 71/2 feet leaders could be used on the small streams. Spools of tippet to match the leader size and dry fly floatant are must have items.