A Federation starbase that the Enterprise is heading for on overdue rest leave when it is diverted to investigate a giant space amoeba in The Immunity Syndrome.
Starbase 6 is a Starfleet facility mentioned in the original series episode The Immunity Syndrome, set in 2268. It does not appear on screen.
At the start of the episode the crew of the USS Enterprise is exhausted after a demanding run of missions and is bound for the starbase for rest and recreation. Before the ship can get there, Spock senses the deaths of the four hundred Vulcans aboard the USS Intrepid, and the base orders Kirk to find out why contact has been lost with both that ship and the Gamma 7A system.
The cause is a vast single-celled organism that drains energy and life, which the Enterprise destroys from within with an antimatter charge. With the creature dead and Spock recovered from his shuttle flight into it, Kirk at last sets course for Starbase 6.