Fieldwork is an important part of biology as well as science and biology education. However, teachers perceive several reasons for the limited use of fieldwork in schools. Further, outdoor education is often organised as a single fieldtrip guided by outdoor educators, and little research has been done on fieldwork as a regular part of formal biology education. This case study explores three secondary-school biology teachers who untypically use outdoor education as a major part of their ecology courses for 8th grade students (median age 14). Berger and Luckmann's theory of the process of institutionalization as a theoretical background is used to interpret the pedagogical and organizational choices of the case study teachers. Analysis of the interviews of the selected three teachers revealed pedagogical and organizational means through which outdoor teaching is institutionalized into a regular activity in biology lessons. The teachers considered regularity, assessment practices and the school curriculum as major tools to legitimate outdoor learning as a formal schoolwork and foster successful learning. However, they also emphasised students' freedom during outdoor activities. The findings are discussed in terms of how the teachers succeeded in combining the institutional order of formal schooling with students' freedom in nature.