This study investigated effect of teaching methods on academic achievement of co-education and single-sex students in senior school biology, using quasi-experimental research design. The sample size was 323 Senior School Two students from 23 Senior Secondary Schools in Yenagoa and Ogbia Local Government Areas, from population of 6,988 Senior School Two students offering Biology as a subject in the Senior Secondary School in Bayelsa State. One research question and one hypothesis were used. Guide on Lecture Teaching Method, Discussion Teaching Method Guide and Guide on Demonstration Teaching Method were reliable and validated guides used by teachers on how to use the methods of teaching. Standardized Biology Achievement Test was adopted as a reliable instrument for data collection. Percentage, mean, standard deviation and z-test statistics were used for analysis of data. The findings revealed that there is no significant difference in the mean scores of SS2 students in single-sex and co-education schools taught with demonstration method in academic achievement in Biology.