An experiment was conducted to examine the mediating role of physiological arousal in social facilitation. It was hypothesized that the elevation of arousal level by the presence of other persons or evaluative apprehension would facilitate task performance. Twenty-four male and 24 female college students performed simple task alone, with a cooperative person, or with a competitive person. One half of the subjects was given an instruction which would produce evaluative apprehension. Subjects' skin potential responses were measured as the indices of physiological arousal. Although the hypothesis was supported by the analysis of psycho-physiological index, but it was not at all by the self-reported index of arousal.