The Barbara series was a group of five hundred identical androids who inhabited the planet Mudd. They were designed according to the personal specifications of Harry Mudd.
The Barbara series was used as a model to describe to the crew of the USS Enterprise some of the engineering specifications of the Mudd androids. Covered with a self-renewing plastic over a skeleton of beryllium-titanium alloy, the duration of this model was estimated at 500,000 years. (TOS: "I, Mudd")
The Barbara series, only identified in dialogue, was established by the Star Trek Concordance to be the series portrayed by uncredited twin sisters Colleen and Morreen Thornton.
Three Barbaras were seen in the episode: #4, #322, and #492.