Many post-secondary institutions provide training and resources to help GTAs fulfil their teaching roles. However, few programmes focus specifically on the teaching competencies required by GTAs who work with undergraduate students in laboratory settings where learning tends to be more active and inquiry based than in classroom settings. From a review of 8 GTA manuals, we identified 20 competencies and then surveyed faculty and lab coordinators (FIS) and GTAs from a Faculty of Science at a comprehensive Canadian university to identify which of those competencies are required of GTAs who work in undergraduate science labs. GTAs and FIS did not significantly differ in the competencies they view as required for GTAs to work effectively in undergraduate labs. But, when comparing the responses of GTAs and FIS to TA manuals, "Clearly and effectively communicates ideas and information with students" was the only competency for which there was agreement on the level of requirement. We also examined GTAs' self-efficacy for each of the identified competencies and found no overall relationship between self-efficacy and demographic characteristics, including experience and training. Our results can be used to inform the design of training programmes specifically for GTAs who work in undergraduate science labs, for example, programmes should provide strategies for GTAs to obtain feedback which they can use to enhance their teaching skills. The goal of this study is to improve undergraduate lab instruction in faculties of science and to enhance the teaching experience of GTAs by better preparing them for their role.