Thirty Hostile and 30 Nonhostile subjects were selected from female Japanese college students based on their responses to the Suspicion and Resentment scales of Buss-Durkee Inventory. The subjects were exposed to opponents in a competitive reaction task. The opponents either increased the intensity of their attack from moderate to high level, remained moderate, or decreased from moderate to low. The dependent measure of physical aggression was the shock intensities which the subjects delivered to the opponents following the procedure of post-contest shock choice. It was observed that Nonhostile subjects' shock choice was governed by the norm of reciprocity, but Hostile subjects did not reduced their shock intensities under decreased attack. This was interpreted as due to Hostile subjects' tendency to attribute a malicious intent to their opponents and to their prolonged inner instigation to aggression despite lowered outer provocation.