As a follow-up to the human-centred and cybernetic structuring principles described by Kattman and Schaefer in /. Biol. Educ. 10 (3), an ecological structur ing principle is discussed. This principle takes the dynamic and reciprocal relationships between man and his environment as the basis of his existence. The reciprocal relationships are characterized as the biological capacity for self-preservation, reproduction, and adaptation. Several examples from a teaching syllabus are given to illustrate this approach.