Co-teaching has gained considerable interest as a means of promoting the inclusion of students with disabilities in mainstream education. Nowadays, there is a consensus among researchers that co-teaching should provide effective education to all students in a mainstream class. This study aims to explore co-teachers' attitudes towards co-teaching responsibilities in Greek mainstream classrooms. In particular, it examines co-teachers' attitudes with regard to their non-class-time (planning and evaluation) and class-time (instruction and behaviour management) responsibilities for students with and without disabilities. Four hundred co-teachers participated in this survey study. Overall, our findings demonstrate that mainstream education teachers and special education teachers disagree about their respective class-time and non-class-time responsibilities, and that their role influences their attitudes towards these responsibilities. The study concludes that the different attitudes of co-teachers towards their responsibilities could hinder the development of a shared approach in teaching students with and without disabilities in inclusive classrooms.