This study investigated the effects of teaching trajectories on students' understanding of difficult concepts in Biology. Two research questions and two null hypotheses guided the study which was carried out in Obio/Akpor Local Government Area of Rivers State. Two public coeducational schools out of thirteen drawn through purposive sampling techniques were used. The instrument for data collection, the Students' Photosynthesis Achievement Test (SPAT) was validated and its reliability coefficient was 0.78. Three intact streams of SS 1 classes in the two schools assigned to book-ending, guided discussion and jigsaws groups were used. The sample consisted of two hundred and thirteen SS 1 Biology students. Mean and standard deviations were used to answer the two research questions while paired t-tests and ANCOVA were used to answer the two hypotheses at 0.05 level of significance. The study revealed that though the three instructional strategies of book-ending, jig-saws and guided discussion enhanced students' understanding of difficult concept in Biology as shown by the improved achievement scores, the jig-saw strategy was the best. It also revealed that there was a significant difference between the achievement of males and females in favour of females in the three groups. Recommendations made based on the findings of the study included that teachers adopt the jigsaw strategy to enhance students' understanding of difficult concepts in Biology.