Previous research reported inconclusive and even contradictory findings on the relationship between locus of control and cooperative behaviour. In the present paper we argue that this is because there is no reason to expect that internals are more cooperative than externals, or vice versa. Going back to the very definition of the locus of control concept, we hypothesise that internals are likely to use both cooperative and competitive behaviour strategically to further their own self-interest. In order to test this hypothesis 39 subjects played three repeated prisoner's dilemma games. Their locus of control was measured by means of Rotter's I-E Locus of Control scale [Rotter, J.B. (1966). Generalized expectancies for internal versus external control of reinforcement. Psychological Monographs: General and Applied, 80, 609 pp.]. Our empirical findings indeed reveal that internals seem to have the adaptive capacity to instrumentally switch from cooperative to competitive behaviour and vice versa, in a prisoner's dilemma game. They also behave more opportunistically when the risk of retaliation is low. The results also make clear that it is essential to study cooperative behaviour dynamically.