This paper reports on part of an ongoing large-scale research on the need to improve science teaching and learning through investigating the Pedagogical Content Knowledge (PCK) of biology teachers for the topic Biodiversity. Six factors have been seen to affect teacher PCK, i.e., content knowledge, knowledge of students, science teaching orientations, knowledge of assessment, knowledge of instructional strategies and knowledge of the curriculum. This research aimed to examine the teacher’s level of content knowledge (CK). A qualitative research paradigm was adopted, and a case study research design used. The case (unit of analysis) was Biology teacher CK, and the subjects were the four teacher participants purposively selected. Lesson observations, teacher interviews and learner questionnaires were used to collect data on teacher CK. A content knowledge analytical framework consisting of five constructs was designed and used to analyse the teacher CK and data triangulated with data collected from interviews and questionnaires. This research revealed that ‘A’ level Biology teachers’ CK vary from teacher to teacher depending on several factors which include teacher identity, planning, workshopping, and motivation among others. Of the four Biology teacher participants, two had adequate CK and the other two exhibited inadequate CK. Inadequate CK was attributed to lack of planning, non-exposure to workshops and lack of teacher motivation. Consequently, this research recommends supervision of teachers from school level to national level, a series of teacher workshops on the demands of the competence-based curriculum and constructive teacher identity as well as introduce factors that enhance teacher motivation. Further research on the content knowledge of Biology teachers in other learning areas is recommended.