Teachers' organizational citizenship behaviour (OCB) is the teacher behaviour that is discretionary, not directly or explicitly recognised by the formal reward system. The factors found to be determining Teachers' OCB are different, hinging on country-culture specific nature. Research studies in the sphere of Teachers' OCB have heretofore been overlooked in Sri Lanka and other similar Asian countries. Hence, this study aims at exploring the degree of prevalence of teacher OCB and its dominant determinants among the teachers in Sri Lanka. The study draws on in-depth qualitative data from interviews and the participants included a convenience sample of teachers and principals employed in secondary schools. The data were analysed deductively using content analysis method. The findings reveal that the teachers' propensity to perform OCB - towards students, school and their colleagues - is on the decline. The determinants, such as work-family conflict, perceived organizational support, teacher values, teachers' self-efficacy, student behaviour patterns, and teachers' pupil control ideologies, seem to be dominant attributing to the low levels of teachers' OCB.