A newly recognized left exoccipital-opisthotic of a Callovian pliosaur, derived from the Peterborough or lower Stewarby Members of the Oxford Clay Formation of Peterborough, is described and figured. This isolated bone is tentatively identified as belonging to an ‘adult’ individual of Liopleurodon ferox that is inferred to have had a skull length of 1.26 metres and an overall body length of 6.39 metres.