This study examines the subjects and concepts in biology perceived to be difficult to learn and teach by 759 pre-service biology teachers registered in the pedagogical formation program at Uludag University Faculty of Education in the academic year of 2005-2016, as well as the associations that word "biology" first calls to their mind. The study was designed as a survey model, and a questionnaire of three scaled questions was used as a data collection tool. The data were classified into 10 different categories based on different branches of biology: Anatomy-Morphology, Biotechnology, Ecology, Evolution, Physiology, Genetics, Histology, Cytology, Systematics and Reproduction and Development. Following the classification, the data were analyzed using frequency and percentages. The subjects and concepts perceived to be difficult to learn by the pre-service teachers are respectively as follows: photosynthesis and its stages, genetics, circulatory system, aerobic respiration, respiratory system, anaerobic respiration, protein synthesis, phanerogams, DNA structure and function, cryptogams, mitosis, plant systematics, endocrine system, meiosis, ETC, urinary system, plant physiology, taxonomy, evolution and digestive system. The subjects and concepts perceived to be difficult to teach by the pre-service teachers are respectively as follows: photosynthesis and its stages, protein synthesis, circulatory system, endocrine system, meiosis, plant systematics, evolution, mitosis, reproduction, genetic crossing over problems, Latinization of names, urinary system, genetics, ecology, aerobic respiration, DNA structure and function, plant tissues, glycolysis and anaerobic respiration. These findings confirm the need for developing undergraduate programs in biology and promoting the research culture at the secondary education level.