Background: Education Standards highlight the importance of conceptual knowledge. Instructional tasks play a key role in biology instruction and offer learning opportunities for students. Tasks contain of a content and a demands part. Therefore, complexity and cognitive level are adequate variables to describe tasks and are assumed effective for students' learning. Purpose: This study aimed to analyse the use of high-complexity and high-cognitive-level tasks in biology instruction and effects on students' factual and conceptual knowledge. Sample: Twenty-eight German secondary school biology teachers were videotaped for three lessons in grade six on the topic botany. The student sample was 701. Design and methods: Instructional tasks were identified and categorised according complexity and cognitive level. Students' factual and conceptual knowledge was tested after the videotaped lessons. Multilevel analyses were used to estimate effects on students' knowledge. Results: Biology instruction is dominated by low-complexity and low-cognitive-level tasks. Multilevel analyses showed a positive significant effect of high-cognitive-level tasks on students' conceptual knowledge. High-complexity tasks did not significantly effect students' conceptual knowledge. Conclusions: Establishing Education Standards did hardly change the type of tasks. The potential of high-cognitive-level tasks to foster students' learning is highlighted. Introducing these results in pre-service and in-service teacher education seems necessary.