The use of effective questions is an essential attribute of successful early years science teaching. In this case study, we analyse the questions and dialogues generated by four preschool teachers from two schools in Buenos Aires, Argentina, of contrasting socioeconomic contexts (one privileged and one underprivileged). We looked at the implementation of two identical enquiry-based science curricular units after a two-month professional development programme. We found that teachers at the more privileged school asked an average of 22% more productive questions, i.e. those aligned with lesson goals (Martens, M. L. 1999. "Productive questions: Tools for supporting constructivist learning." "Science and Children" 36 (8): 24-53. http://search.proquest.com/openview/0fbd77105695538253f998eae1d38ff4/1.pdf?pq-origsite=gscholar&cbl=41736). There were also significant differences between the types of questions asked, with teachers from the lower socioeconomic status school asking more 'attention-focussing' and fewer 'evaluation' questions. Qualitative analysis of the teacher-student dialogues showed that students from the privileged school were exposed to higher-quality learning experiences, despite teachers apparently performing the same activities with the same materials. Our findings highlight the importance of focussing early years science professional development efforts on questioning practices, especially for those teachers working in underprivileged settings, to foster meaningful learning opportunities for all children.