The author has used special preparation techniques and phase contrast microscopy for the study of the normal and the pathologically changed cochlea of guinea pigs and squirrel monkeys. A short description of this technique of making surface preparations of the organ of Corti will be given, as well as a presentation of figures showing the outstanding way in which it permits in one specimen the analysis of damage in every single one of more than a thousand consecutive sensory cells. The surface preparation has replaced the preparation techniques earlier used by the author for several purposes and is characterized by its simplicity and reliability.