Students' ability beliefs about running activities in physical education/athletics classes were investigated longitudinally from the perspectives of Nicholls' conceptions of ability and Dweck's implicit theories of ability. The data for this study were provided by 44 students (20 boys; 24 girls) who participated in a larger longitudinal project on students' running motivation in schools. They were individually interviewed from fourth to eighth grade. All interviews were tape-recorded and transcribed verbatim for inductive analysis, revealing the criteria that students used to assess their own ability in running and reasons why they believed they could improve their ability in running. Frequencies and percentages were computed to determine changes in students' ability assessment criteria over time. Results showed that students used ability, effort, task mastery, intra-individual comparisons, and social comparisons as criteria to assess their own ability in running as they progressed from the fourth to eighth grade. However, they increasingly used social comparison in assessment as they became older. Most students believed they could improve their ability in running, suggesting that they were incremental theorists. Ability/maturation, effort, experiences, and influences from significant others were cited as contributors to their beliefs about improving running ability. This study took the first step to provide longitudinal qualitative data on students' ability beliefs about running in physical education/athletics classes. Findings of the study revealed students became more concerned with social comparisons in their ability assessments over time and held a belief that they could improve their own running ability. The implications of these findings for physical education teachers are discussed.