"Reciprocity" is the eleventh episode of the third season of the American science fiction drama television series Fringe, and the 54th episode overall. In the episode, the Fringe division follows a chain of shapeshifter murders as the assembly of the doomsday device nears completion. Ryan McDonald and Charles Parnell guest starred.Co-executive producer Josh Singer wrote "Reciprocity", his seventh such credit for the series.