From TBhttp://www.talkbass.com/forum/f28/seinfeld-theme-bassplayer-17234/"In Bass Player Magazine's 10th Anniversary issue, in the article "Will Lee's 10 Favorite TV Themes That Feature Cool Bass Lines", Jonathan Wolf is credited with the Seinfeld theme. He was the musical director on The Arsenio Hall show. 10. Northern Exposure (David Schwartz)09. Barretta, "Keep Your Eye on the Sparrow" (Chuck Rainey)08. S.W.A.T. (Scott Edwards)07. Fat Albert (Unknown)06. Seinfeld (Jonathan Wolf)05. Ironsides (Chuck Rainey)04. Sanford & Son (Chuck Rainey)03. Courageous Cat (Unknown)02. Barney Miller (Jim Hughart)01. Peter Gunn (Unknown)"
I believe that's a keyboard.