An experiment was performed in which subjects engaged in a "Path and Obstacles" game where the subject had the task of tracing a path on a 5 x 5 plugboard in such a way as to avoid hitting any of the five obstacles that had been placed by the other player. Half the Ss were told that the other player was a machine which had been programmed to play the game and the other half were led to believe that they were playing against another human. In addition, half the players were told to compete against the other player while half were told to cooperate. In every instance the setting of the obstacles by the other player was determined by a prearranged plan. It was concluded that a subject's expectations concerning the other player are a critical element in determining whether his or her perfomance on the task will be affected by a "human" or "machine" other. It was also tentatively concluded that "human" others induced more irrationality than "machine" others.