Participants completed blocks of trials under simple (i.e., work-alone), competitive, and self-competitive fixed-interval 20-s schedules of reinforcement. In general, response rates were highest during competition and lowest while working alone. In addition, whereas participants emitted responses at a constant rate while working alone, competitive responses often followed a break-and-run pattern; self-competitive responses, in contrast, were highly variable in both rate and pattern, often changing within a single session. Finally, no systematic relation was found between rates of responding and probability of reinforcement under competitive and self-competitive conditions. These observations suggest that self-competitive behavior may contain many of the characteristics of competitive behavior. (Contains 2 tables and 7 figures.)