In science education, dissections of animals are an integral part of teaching, but they often evoke negative emotions. We aimed at reducing negative emotions (anxiety, negative affect [NA]) and increasing positive affect (PA) and self-efficacy by an experimental intervention using a predissection video to instruct students about fish dissection. We compared this treatment with another group that watched a life history video about the fish. The participants were 135 students studying to become biology teachers. Seventy received the treatment with the dissection video, and 65 viewed the life history video. We applied a pre/posttest treatment-comparison design and used the Positive and Negative Affect Schedule (PANAS), the State-Trait-Anxiety Inventory for State (STAI-S), and a self-efficacy measure three times: before the lesson (pretest), after the film treatment (posttest 1), and after the dissection (posttest 2). The dissection film group scored higher in PA, NA, and state anxiety (STAI-S) after the dissection video treatment and higher in self-efficacy after the dissection. The life history group showed no differences between the pretest and posttest 1. The dissection film has clear benefits - increasing PA and self-efficacy - that come at the cost of higher NA and higher STAI-S.