Yellowstone National Park
Pets are allowed in developed areas of the park. This includes any areas within 100 feet (30.5 meters) of roads, parking areas, and campgrounds (excluding thermal areas).
Pets must be kept under physical control at all times - caged, crated, or on a leash not to exceed six feet in length.
Pets are not allowed on trails or boardwalks, in the backcountry, or in thermal areas. Thermal areas pose particular hazards to pets.
For more information about Yellowstone National Park see here.
Before You Go Exploring in the USA
For a discussion on state law regarding cats, see here.
Most locations require pets to be on a leash less than 6 feet long.
Typically pets are not permitted in public buildings.
Cats are welcome:
in developed areas
on many trails and campgrounds
in some lodging facilities
Cats must be restrained, either on a leash (not exceeding 6 feet in length) or caged/crated.
Pets are not permitted:
in public buildings
designated swimming beaches
Cats are allowed in national forests, but must be kept on leash (less than 6 feet in length) while in developed recreation areas and interpretive trails.
Cats are not permitted in swimming areas.