Cat Ba is the largest island in the Bay and approximately half of its area is covered by a National Park, which is home to the highly endangered Cat Ba Langur. This golden-headed langur is rarely seen, as fewer than 100 specimens are thought to survive in the wild, although it is the subject of a well-organised conservation programme. The Park covers both land and marine areas and has a high biodiversity, although it is at risk from too rapid an increase in tourism. Other mammals in the Park include civet cats and oriental giant squirrels.
Many tour operators include an option of trekking in the National Park or canoeing on three-day tours; shorter tours generally only stay overnight in the small town of Cat Ba (population about 8,000) or on boats moored in Cai Beo bay, about 2km away from Cat Ba town. Cat Ba itself is attractively situated around a bay teeming with small boats, many of which belong to pearl or shrimp farmers, and can become very busy at weekends and during public holidays. The promenade has illuminations and a large fountain which only plays after dark; it is backed by a strip of cheap hotels and bars, but dominated by the wooded limestone hills behind. The island is a national park of Vietnam and was recognized by UNESCO in December 2004 as a Biosphere reserve of the world.