14 December 1961
On 14 December 1961, Tanganyika became a member of the United Nations(UN) and two years later, in 1963 it merged with the island of Zanzibar. Following the historical merger between the two countries (Tanganyika and Zanzibar) their membership status at the UN changed to a single member state on 26 April 1964. Subsequently, on 1 December 1964 the two countries became known as the United Republic of Tanzania. Julius Kambarage Nyerere became the first President of the United Republic of Tanzania and Sheikh Abeid Karume became Vice President. Tanganyika’s membership to the UN came five days after the country gained independence from Britain on 9 December 1961. At the time Nyerere was a Prime Minister after winning the August 1960 elections by 70 seats out of 71 in the Tanganyika Assembly. The United Republic of Tanzania participates in the following principal organs of the UN: General Assembly, Security Council, and the Economic and Social Council.
References

Tanzania-un.org (2011). ‘Tanzania at the United Nations’from Permanent Mission of the United Republic of Tanzania to the United Nations [online]. Available at www.tanzania-un.org [Accessed 09 November 2011]|Vpo.go.tz (2011).‘Background history of The Union between Tanganyika and Zanzibar’from The Vice-Presidents Office [online]. Available at www.vpo.go.tz [Accessed 09 November 2011]|Encyclopaedia Britannica (2011).  ‘Julius Nyerere’from Encyclopaedia Britannica’s Guide to Black History [online]. Available at www.britannica.com [Accessed 09 November 2011]