This study investigated the effect of class size on the academic performance of biology students in senior secondary school in Lagos state. The major purpose of the study was to find the impact of class size on the academic performance of biology students. The study was a causal comparative research design. The sample of the study comprise of two hundred and ninety-five (295) selected biology students and 4 biology teachers. The instruments of data collection used for this study are second term results of biology students from selected state-owned schools in Lagos state and open-ended questionnaires structured by the researchers. Face and content validity of the instrument was carried out by giving a copy of the instrument to 3 experts in the field of biology education. This study had five (5) research questions and three (3) research hypothesis. Descriptive statistics (the frequency count and percentages method, mean, standard deviations) was used to analyzed the research questions, while independent sample t-test was used to test the research hypothesis. From this study, it was discovered that large class size has a negative effect on biology students academic performance, also findings from the study revealed that large class size has no influence on gender difference and that large class size has impact on the ability level of biology students. This study recommended that Lagos state government through her educational agencies should provide more classrooms, enough teaching and learning materials, educational facilities to ensure effective teaching and learning of biology and to improve the academic performance of biology students.