An experiment on the action of the yeast, "Saccharomyces cerevisiae", against a fungal plant disease is proposed for secondary students (Grade 11) to support their study of organism interrelationship. This biocontrol experiment serves as the basis for discussing relationships among three organisms (red chilli fruit, "Saccharomyces cerevisiae," and "Botrytis cinerea"). Students have the opportunity to observe the different types of cell structure and function of these organisms. The ability of the yeast to control mould growth was detected by "in vitro" and "in vivo" tests. Competition for nutrients (glucose and fructose) between yeast and mould were also demonstrated. In addition to cognitive learning, this simple experiment can provide manipulative skills for students' further investigation in their science projects and later in higher education. (Contains 3 figures.)