Although the central role of biology in nation’s scientific and technological advancement is unquestionable, the academic achievement of students in the subject has been subpar. To cope with this situation, researchers in science education have continued to look for a successful way to teach and learn biology concepts. The present study, therefore, investigated the comparative effects of concept mapping (CM) and cooperative mastery learning (CML) teaching strategies on students learning outcomes in photosynthesis. A non-equivalent pre-/post- test quasi-experimental research design was adopted. 295 secondary school form two students from four co-educational schools made up the sample of the study. Data were generated through photosynthesis achievement test (PAT). PAT was subjected to reliability analysis using Kuder-Richardson 21 (KR-21) and Cronbach’s alpha, which yielded a reliability coefficient of 0.82 and 0.95, respectively. Data were analyzed using mainly descriptive statistics, and inferential statistics of analysis of covariance. The findings revealed that both CM and CML enhanced students’ achievement (F<sub>[1,294]</sub>=26.165, p< 0.05), and the retention of the concept of photosynthesis (F<sub>[1,294]</sub>=9.042, p< 0.05). However, CM was significantly more effective than CML. As a result, the recommendation that came up among others was that science teachers should embrace CM in biology teaching as well as other perceived difficult topics in science subjects. Also considered important was to train science trainee teachers to use CM.