Summary : The contrafreeloading phenomenon. The contrafreeloading phenomenon corresponds to the following experimental situation : animals having been exposed to a certain conditioning, of operant behaviour for instance, go on pressing the lever as they have been trained to respond, even when a cup full of pellets of food has been placed in their box. The theoretical interest of this kind of study is that it apparently calls in question the validity of the law of least effort. An analysis of the experiments which have been conducted since the discovery of the phenomenon demonstrates the complexity of the variables which determine the behaviour of animals in such situations. Key words : contrafreeloading, law of least effort, operant conditioning.