Liam Sullivan (18 May 1923 – 18 April 1998; age 74) was an actor who played Parmen in the Star Trek: The Original Series third season episode "Plato's Stepchildren".
Sullivan studied drama at Harvard and moved to New York City after graduation, where he made his television debut in 1949 and his Broadway debut in 1951. In the next four decades, he gained over 120 television credits, including the memorable The Twilight Zone episode "The Silence" with Felix Locher and Arthur Tovey).
He also guest-starred on series such as Alfred Hitchcock Presents (with Brian Keith), Wagon Train (with Antoinette Bower), Perry Mason (with Benjie Bancroft and Dick Cherney), The Man from U.N.C.L.E. (with Abraham Sofaer, Yvonne Craig, David Hurst, Nancy Kovack, Mickey Morton, and Bill Quinn, directed by James Goldstone), The Fugitive (with Meg Wyllie, Garry Walberg, and Hal Lynch, directed by Robert Butler), Voyage to the Bottom of the Sea, Lost in Space, Barnaby Jones (with Lee Meriwether, Glenn Corbett, Dallas Mitchell, Bob Hoy, and Ed Fury, directed by Larry Dobkin), Mannix (with Nancy Kovack, Paul Sorensen, and Jack Donner), Starsky & Hutch (with Nick Borgani, Monty O'Grady, and Arthur Tovey), Little House on the Prairie (with Phillip Pine), Magnum, P.I. (with Branscombe Richmond), Dynasty (starring Joan Collins, with Peter Mark Richman, Clive Revill, and Robert Hitchcock), The A-Team (starring Dwight Schultz and Melinda Culea, with Nick Dimitri and Richard Lynch), St. Elsewhere (starring William Daniels, Norman Lloyd, Ed Begley, Jr. and Christina Pickles, with Kavi Raz and David Ruprecht), L.A. Law (starring Corbin Bernsen, with Mimi Kuzyk, Bibi Besch, Anne Haney, Denise Crosby, and Josh Clark), and Dallas (with Morgan Woodward).
Sullivan also appeared in an episode of the 1986 miniseries North and South, Book II, alongside Kirstie Alley, Jonathan Frakes, Mary Crosby, James Read, Jean Simmons, Anthony Zerbe, David Ogden Stiers, William Schallert, Beau Billingslea, and Kurtwood Smith.
His very few feature film roles include The Magic Sword (1962, co-starring with Gary Lockwood).
Sullivan died of a heart attack on 19 April 1998. (Star Trek Monthly issue 43, p. 8)