Reform-minded practices are widely encouraged during pre-service science teacher education in concert with national reform documents. This contrasts to the nature of instruction within university science laboratories in which pre-service teachers enrol, which are largely confirmatory in nature. Undergraduate science laboratories are taught predominantly by graduate teaching assistants (GTAs) with minimal teacher preparation. The purpose of this mixed-methods study is to investigate the instructional practices and teacher beliefs of eight GTAs at a university with very high research activity who completed a reform-minded Teacher Certificate Programme, asking: What are their beliefs about teaching? How are their practices described? Do their beliefs and practices differ from one another? Do their teaching beliefs correspond with their practices? Findings indicate that GTAs held moderately reform-minded "transitional" beliefs of teaching following the programme, yet displayed fairly traditional instruction. Cross-case findings highlight similar patterns across subscales of the RTOP that draw attention to underlying constraints of the laboratory curriculum structure. We suggest that GTA professional development is best undertaken concurrent with laboratory course revision. (Contains 14 tables and 2 figures.)