IN A RECENT editorial on the implications of Piaget's research for biology teaching, Creager (1975) noted that need to provide opportunities for students to be involved in planning some of their own learning Creager also noted that cognitive development occurs accommodation is required, when the student is engaged in solving problems involving conflicting evidence or inconsistent data. The classroom laboratory experiment described in this article provides both of these opportunities. The experiment gives students opportunities to reason as scientists do. When students have such opportunities, they come to appreciate the excitement of scientific investigation and to experience the truly creative aspects of the scientific enterprise. In order to achieve these goals, effective biology teachers frequently seek out new open-ended laboratory investigations that will provide students with such experiences. Genetics is a discipline ideally suited to introducing students to the experimental study of living organisms. Drosophila melanogaster, the vinegar or fruit fly, is an organism with which most biology teachers have had some classroom experience, either as teachers or while they, themselves, were students. Consequently, an open-ended genetics laboratory investigation employing drosophila can be useful to a large number of secondary school and college biology teachers. As some readers will no doubt recognize, this is not the first occasion on which I have extolled the virtues of using drosophila in investigatory laboratory exercises (Gardner and Mertens 1975; Mertens 1969, 1972). I am thoroughly convinced that such experiments are useful not only in teaching basic principles of genetics but also in enabling students to experience the delight of scientific investigation in truly experimental situations. The purpose of this article is to describe a simple drosophila experiment which both my students and I have found challenging. I became fascinated with this experiment when we first used it in the classroom laboratory. The experiment has at least two virtues. First, the deceptively simple data obtained in the experiment are consistently repeatable; the experiment really works. Second, the students in my classes have been able to suggest three different but biologically sound hypotheses that will satisfactorily explain the data obtained. There does not seem to be just one correct ''answer to the problem! Having three reasonable explanations sets the stage for developing studentdesigned experiments to differentiate between these hypotheses. Thus, the student collects data and formulates, tests, and refines hypotheses as does any research scientist. In my opinion, this experiment has much to offer the teacher wishing to give his students such experiences. The biology teacher must be willing to use a little ingenuity in preparing for the laboratory experience to be described below. Mutant names must be coded so as not to provide clues as to the mechanisms of inheritance involved. Students must be placed in a situation in which they formulate hypotheses and draw conclusions based on experimental data and not on subtle clues provided by the instructor. Thus, the instructor must describe the fly stocks entirely in terms of phenotypes; it is the students' responsibility to ascertain genotypes on the basis of experimental results. To follow the advice just given, the experiment will first be described in terms of what the student in the class perceives when the P,, F,, and F2 generations of flies are observed. Thus, the phenotypes of flies will be described in everyday terms without using the officially designated descriptive words the drosophila geneticist would use. This description will be followed by the presentation of three alternative hypotheses, all of which will adequately explain the data. First, the verified hypothesis will be presented; this hypothesis explains what is actually happening as the traits are transmitted from parents to F1 to F2. As the instructor who designed the experiment, I was aware of this hypothesis before the data were obtained. My students, however, had no such preconceived hypotheses. Second, two