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 60 specimens are thought to survive in the wild, although it is the subject of a well-organised conservation programme; the Cat Ba Langur Conservation Program, an international NGO that has its office at the National Park Head Quarters. 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.

Outside of Cat Ba town is a beautiful island with much to explore.