One of the most important elements of formative assessment is 'identifying the learning gap'. This study aimed at examining biology teachers' practices of formative assessment via the 'identifying learning gap' element; The sample of the current study consisted of a total of 27 biology teachers drawn from the city of Erzurum, Turkey. The data from semi-structured interviews were exposed to content analysis. The results showed that even though the teachers focused on the students' pre-existing knowledge at the beginning of the course, they disregarded their individual differences and alternative conceptions that would influence their learning. Also, it was elicited that they were pitfalls at individual assessments and learning difficulties of their students. In light of the results, the present study recommends that biology teachers be fostered to attend pre-service and in-service education for formative assessment.