This case explores two-species interactions, especially mutualism, and presents students with a problem, namely, the inconsistent treatment of the concept of mutualism and symbiosis in many textbooks. It begins with a question that students will probably not feel qualified to answer: Is the equation of mutualism and symbiosis in a textbook correct? It then guides students through the concepts of two-species interspecific interactions, and returns to the central question. After exploring the case problem thoroughly, it ends with some thoughts about evolutionary dynamics. The case was designed for an introductory ecology course or a science education course. It can also be used with slight modifications in an introductory biology course.