The purposes of the study were to explore how the experiences that pre-service chemistry teachers have during one-semester practicum influence Teaching Efficacy Beliefs (TEB) and to identify the sources of TEB during the practicum. Twenty-six pre-service teachers were the participants. The practicum included teaching experience in high school and microteachings at the college of education. Teacher's Sense of Efficacy Scale (TSES) was administered at the beginning and at the end of the practicum. Six of the participants selected purposefully were interviewed about TEB development and sources of it. Results showed that pre-service teachers' TEB slightly increased through the end of practicum; however, it was not uniform for all TSES sub-scales. Classroom management was especially problematic for the participants. Mastery experience in high school and during microteachings, vicarious experience in microteachings, and mentors' persuasion were sources of TEB. Implications were provided for pre-service teacher education.