Ten chinchilla cochleas which ranged in length from 16.00 to 19.71 m were used for this study. The cross-sectional area and perimeter of the inner and outer hair cells and their nuclei were determined at two locations per cochlear turn and at the junction between each of the turns (11 locations from apex to base). Some interanimal variation in hair cell dimensions was noted. However, none of these size variations could be attributed to any known differences among the animals. It was concluded that the data reported here represent the natural variation in hair cell size in chinchillas. The data establish a baseline to which the dimensions of cells from abnormal cochleas can be compared, regardless of the length of the cochlea or the base-to-apex location of the cells.