The structure of the cochlea had been studied by serial sections in 13 species of mammals, with representatives from the metatherian order Marsupialia and the eutherian orders Insectivora, Primates, Edentata, Pholidota and Carnivora. All the specimens have been intra-vitally perfused to obtain good preservation of the cochlea. The height of the cochlea was measured and the number of half-turns noted in each cochlea. Other measurements were made at every half-turn of the cochlea in the modiolar plane as follows: the width of the cochlea and the radial position of the spiral organ, the width and thickness of the basilar membrane, the height of the cells of Claudius and the size of the spiral ligament. Other special features were also observed.