School subjects divided into compulsory and elective courses. Compulsory courses are the basic courses that students must take to graduate, while elective courses give freedom for students. In Turkey, Biology courses taught as compulsory under the required courses in high schools. But, it is also instructed as elective courses called "Advanced Biology" in Anatolian High Schools and "Biology Applications" in Science High Schools at grades eleven and twelve. This new development calls for determining high school students' reasons for choosing and not choosing biology. To elicit their reasons, the sample of the current study consisted of 135 volunteer students drawn from two different schools in northeastern Turkey. An open-ended questionnaire used for data collection. The students' responses to the survey subjected to content analysis. The factors affecting their choices of biology were high due to future-oriented issues and emotional reasons. The factors affecting students' views of declining biology as an elective course were high due to future-oriented issues, psychological reasons, and external resources. They tended to change their decisions because of related-courses and exam-oriented reasons. Future studies should be undertaken with larger samples to identify the role of gender differences in biology preferences.