Introduction: One of the aims of physical education (PE) is to develop social skills such as cooperation, teamwork, and mutual helping among students. Cooperation is a broad research topic, implicating several disciplines in the human sciences (e.g. psychology, sociology, linguistics, philosophy). It is also an important topic in various domains of practice like organizational management, ergonomics, sports performance, and PE and sports pedagogy. Studies in sport have shown that cooperation between partners is not automatically produced by the cooperative structure of the learning tasks. In this exploratory study, we focused on the links between cooperation and trust judgments about one's partner. We characterized the processes by which students construct trust judgments in dyadic cooperative interactions during climbing lessons. This study was carried out within the theoretical and methodological frameworks of the 'course of action.' Method: Two middle-school students in 10th-grade participated in this study. They formed a fixed dyad based on personal affinity. During the four climbing lessons under study, the students alternated the roles of climber and belayer. Their behaviors and communications were recorded on video and after each session they participated in self-confrontation interviews. From these data, we then reconstructed their courses of experience and focused particularly on preoccupations, meaningful elements in the situation, and mobilized knowledge. The elements contributing to the construction of the climber's trust judgments about his belayer were identified. Changes in these judgments were characterized in relation to the modes of cooperation between the students. Twenty-seven climbs were analyzed. Results and discussion: The results showed that the climber's trust judgments focused on two dimensions of the belayer's activity: the reliability of both the belay and his advice for success in the task. To build these trust judgments, the climber combined knowledge about his belayer (the partner's climbing skills, his mastery of safety techniques, his familiarity with the climbing equipment, and his typical attitudes in class) and interpretations of the events during belayer--climber cooperation (the attention shown by the partner, his requests for advice from other students, and the teacher's interventions). Moreover, the results showed the relationship between the development of each student's trust judgments about his partner and the positive and negative dynamics of cooperation between the students. Three typical connections were identified between the climber's trust judgments about his partner and the climber's involvement in cooperation. The results are discussed on the basis of two points: (a) the dynamic and composite character of the construction of trust judgments and (b) the construction of trust judgments as a condition for genuine cooperation between students. The students in the situation of climber displayed typical processes to build a trust judgment about their partner, and this trust judgment appears to be an important element in promoting and regulating cooperative interactions between students. Conclusions: We propose pedagogical perspectives for teachers with regard to understanding trust judgments in climbing and student cooperation. We also suggest new research perspectives with the objective of fully elucidating the dimensions of trust involved in cooperation.