The aim of this study was to identify the content of secondary Science curriculum excluded in teaching and learning activities, the reasons behind the omission and its impact on students' learning in the context of Bangladesh. This study used qualitative methodology. Eight teachers were selected and interviewed to investigate what they excluded from their teaching and what the reasons behind the exclusion were. Data were collected from participating teachers' students using a focus group discussion. Data collected from students and teachers were analyzed first. Then purposefully selected curriculum developers, textbook authors and teacher educators were interviewed to understand the impact of the null curriculum on students' learning. Analysis of data revealed that all the participant teachers excluded a chapter in grades IX-X General Science textbook. This chapter included information about reproductive organs, physical and psychological changes during puberty, pregnancy, evolution and related topics. Teachers argued that the topics were uncomfortable for them to teach, contradicted with religion, and students might misinterpret and practice unethical relationships if these topics were taught. However, the curriculum developers and the textbook authors asserted that including these topics would solve problems like adolescent mistakes, child rape, child pregnancy and distorted fantasies. This study concluded that excluding the chapter on adolescence, sexual and reproductive health and evolution obstructs achievement of students' scientific literacy.