On trails, including the trail to Vernal Fall (however, pets are allowed on the Wawona Meadow Loop)
On unplowed roads covered in snow
In undeveloped and wilderness areas
In public buildings
On shuttle buses
In lodging areas
In all walk-in and group campgrounds/campsites, including Camp 4
In any other areas, as signed
These regulations protect both pets and wildlife from disease and each other. The National Park Service has prohibited pets on trails for many years. In particular, some pets chase wildlife, pollute water sources, and can become defensive and dangerous in unfamiliar surroundings. Pet owners have the burden to assure their pet does not damage the park values for others in those areas where pets are allowed.
Yosemite Hospitality operates a dog kennel in Yosemite Valley from approximately late May through early September (closed in 2019). Written proof of immunizations (rabies, distemper, parvo, and Bordetella) must be provided. Dogs must be at least 20 pounds (smaller dogs may be considered if you provide a small kennel). You can get more information about the kennel by calling 209/372-8326.
There are a few additional (very obscure and unsigned) places where pets are allowed:
Wawona: Wawona Meadow Loop, Chowchilla Mountain Road, and Four Mile and Eleven Mile fire roads (but not the Four Mile Trail in Yosemite Valley).
Hodgdon Meadow: Carlon Road from the trailhead to Hodgdon Meadow and on the Old Big Oak Flat Road from Hodgdon Meadow to Tuolumne Grove parking lot.