Case studies to investigate the conceptions of teaching science held by three novice teachers participating in an alternative secondary science teacher certification program were conducted, along with the relationships between their conceptions of science teaching and their science teaching practice. Data used to build the cases included the participants' responses to two surveys and transcripts of audio taped interviews with the participants and their university supervisors who observed them teach. Findings revealed that novice teachers hold ideal and working conceptions of teaching science, the novices' conceptions can serve as referents for their classroom practice, and the conceptions of teaching science held by the novice teachers were resistant to change, even in the face of well-intentioned instruction.