At twice the size of Denmark, Selous (55,000 sq km) is the second largest 'park' in the world, said to be home to over one million mammals.
Of this vast wilderness only the Beho Beho region, north of the Rufiji River is open to tourists.
The best time is July-October, though precaution needs to be taken since the area is infested with the Tsetse.

Remote Ruaha, covering some 12,950 sq km, 625 kms from Dar es Salaam, derives its name from the mighty Ruaha river that forms part of its South-eastern boundary.
Ruaha can only be visited in the dry season July-December.The park's altitude (750m - 1,900m) creates four different flora zones: river valleys, savannahs, moimba woodland and undulating bus country.

Part of a far larger migratory ecosystem (that includes Lake Manyara) and dependant on the Tarangire River that flows through its centre, this National Park covers an area of 2,600 sq km and is about 120 km from Arusha. Larger mammals vacate Tarangire in a mass exodus during the annual October and April rains, so the park is best visited once the grasses dry up, from July to September and the river is again in demand.