In sport psychology, there is broad interest in cognitive factors that affect sport performance. The purpose of this research was to examine one such factor, self-talk, in competitive sport performance. Twenty-four junior tennis players were observed during tournament matches. Their observable self-talk, gestures, and match scores were recorded. Players also described their positive, negative, and other thoughts on a postmatch questionnaire. A descriptive analysis of the self-talk and gestures that occurred during competition was generated. It was found that negative self-talk was associated with losing and that players who reported believing in the utility of self-talk won more points than players who did not. These results suggest that self-talk influences competitive sport outcomes. The importance of "believing" in self-talk and the potential motivational and detrimental effects of negative self-talk on performance are discussed.