This qualitative study explored current teachers’ formative assessment practices in Biology classrooms and how these practices influence performance in the subject. The study employed a case study design in a single selected community ordinary-level secondary school in Moshi municipality. It purposively involved two Biology teachers and twenty students selected from classes that do not have national examinations. Data were collected through semi-structured interviews with teachers, classroom observations, and focus group discussions with the students, as well as a review of students’ exercise books. The data were transcribed verbatim, then coded and themes generated through interpretive phenomenological analysis. The findings showed that teacher’s oral questioning dominated the assessment practices. Other strategies such as peer assessment, student-student interactions, descriptive feedback, and sharing of learning intentions with students seemed to be unfamiliar to the Biology teachers involved. The study recommends among other things, in-service training for teachers on how to employ alternative formative assessment practices during instruction to promote students’ learning and achievement in Biology.