SUMMARY
Students' low performance in biology perturbs many teachers and other stakeholders. This has sparked research into learning strategies that might be utilized to improve students’ performance in the subject. This study investigated the effects of Concept Mapping (CM) and Cooperative Mastery Learning (CML) on students' achievement in photosynthesis. A pre-test, post-test, non-equivalent control group, quasi-experimental design was used. A sample of 449 students was drawn from the population of 6,708 senior two secondary students (SS2) in Nyamagabe district, Rwanda. Photosynthesis Achievement Test (PAT) (KR-21 = .82), Attitude Towards Concept Mapping Questionnaire (ATCMQ), and Attitude Towards Cooperative Mastery Learning Questionnaire (ATCMLQ) were used for data collection. Data were analyzed mainly using mean and standard deviation, Analysis of covariance (ANCOVA), and t-test. Findings revealed that the students who were exposed to the CM and CML attained significantly higher mean achievement scores in the PAT than those exposed to Conventional Teaching Methods (CTM) (F (2, 445) = 385.242, p<0.05). The CM group students achieved significantly better than the CML group. Moreover, there was no significant interaction effect between instructional strategies and gender in achievement (F (2, 442) =. 344, p > .05). The students showed a significant and positive attitudes towards the use of CM over the use of CM (t=5.8, p<.05). Thus, the CM and CML are effective strategies for successful and meaningful photosynthesis learning. It was recommended among other things that the biology teachers should use the CM and CML to enhance students’ achievement in biology especially in difficult and abstract concepts like photosynthesis.https://doi.org/10.26803/ijlter.21.2.7