Teaching outdoors is an obligatory part of physical education (PE) in most countries; therefore, PE holds great potential to contribute to learning about nature, the environment and environmental sustainability (ES). This paper reports the investigation of whether Swedish PE teachers perceive ES as part of their subject, and if so, how do they implement it? Drawing on 60 PE teachers' answers to five opened questions (53 written answers and 7 oral interviews), and using British sociologist Basil Bernstein's concepts of classification and codes, this study have asked the following research questions: 1. What influences the position and legitimisation of ES in PE? 2. How does the subject culture of Swedish PE influence the introduction of ES? The result from the study shows a complexity regarding the strength of the boundaries that surround ES in PE and what this complexity might offer in terms of enablers and constraints. The result also displays a strong connection between outdoor education and its potential for teaching ES. Various options are discussed with regard to where (in what content areas in PE) and how to implement ES.