In this article, the role of young children's emotional practices in science learning is described and analyzed. From the standpoint of performativity theory and social-constructionist theory of emotion, it is argued that emotion is performative and the expression of emotion in the classroom has its basis in social relationships. Arising from these relationships is the emotional culture of the classroom that plays a key role in the development of classroom emotional rules as well as the legitimation of science knowledge. These relationships are reflected in two levels of classroom dialogue: talking about and doing science, and expressing emotions about science and its learning. The dynamics of the negotiations of classroom emotional rules and science knowledge legitimation may dispose students to act positively or negatively toward science learning. This analysis is illustrated in the experiences of a teacher and her students during a 3-year ethnographic study of emotions in science teaching and learning. This research suggests the importance of the interrelationship between emotions and science learning and the notion that emotional practices can be powerful in nurturing effective and exciting science learning environments. © 2004 Wiley Periodicals, Inc. J Res Sci Teach 41: 693–719, 2004