This paper comprises a presentation of the findings of a case study that investigated how situational factors triggered 12th grade students' interest during a field trip to a zoo. The purpose was to identify sources of interest and to investigate the attributes that make them interesting. Students' interest was investigated by a descriptive interpretive approach, based on data from classroom and field trip observations, video recording, and interviews. The findings provided evidence that substantial situational interest can be generated during a fieldtrip to a zoo. Students' interest was triggered by variables such as active involvement (hands-on activities), novelty, surprise, and knowledge acquisition (activities and provided information), and social involvement (opportunities for socialisation). The results show that strong interest is stimulated when several variables are in play simultaneously. The study implies that zoo visits can provide students with affective experiences, which can be a powerful way to stimulate students' learning motivation.