As a fundamental movement skill and a life-long fitness activity, running plays an important role in elementary physical education curricula. Little information is available concerning children's ability beliefs about this activity. Research consistently demonstrates that children's ability beliefs influence their motivation, behavior, and performance. Therefore, to effectively promote running among elementary school children, it is important for teachers and researchers to understand their beliefs for running. The purpose of this study, then, was to examine: 1) what sources of information children used to judge if they were good or poor at running and 2) how they thought they could improve their ability in running. Participants were 19 (9 boys; 10 girls) fourth graders from an elementary school. They were randomly selected from a sample of 119 fourth graders who participated in a longitudinal project on children's motivation and performance in a running program. Near the end of the program, the participants were interviewed individually and were asked to respond to questions such as “Were you good at running? Why did you think you were good at it?” “What helped you do well at running this year?” and “Would you think that you could do better at running next year? Why or why not?” Inductive analysis and constant comparison techniques were used to analyze the interview data. Results indicated the fourth graders primarily used five sources of information to judge their ability in running. They included: objective information (e.g., the number of the laps run/walked over the school year), effort (e.g., I tried my best), social comparison (e.g., I ran faster than others), significant others' feedback (e.g., My teacher told me that I was good at running), and natural ability (e.g., I have strong legs). These findings are consistent with the research literature on the formation of children's ability conceptions. Furthermore, 16 out of 19 participants believed that they could do better in running next year. These students believed that they could achieve such ability improvement if they 1) were interested in running, 2) worked hard, 3) received support and encouragement from teachers, parents, and friends, 4) set goals for running, and 5) physically matured over time. These results suggest that in order to maintain children's beliefs that they can continue to improve their ability and performance in physical activity teachers should present activities in interesting ways, value student effort, provide encouragement, and set goals for their students.Keyword(s): elementary education, physical activity