The Collaborative International Dictionary of English v.0.48:

behaviourist \behaviourist\ n.
   same as behaviorist.

   Syn: behaviorist.
        [WordNet 1.5] behaviorist

WordNet (r) 3.0 (2006):

behaviourist
    adj 1: of or relating to behaviorism; "behavioristic psychology"
           [syn: behavioristic, behaviorist, behaviouristic,
           behaviourist]
    n 1: a psychologist who subscribes to behaviorism [syn:
         behaviorist, behaviourist]