The basilar membrane of Pteronotus p. parnellii was studied by light and scanning electron microscopy in order to examine the relationship of membrane structure to the sharply tuned sense of hearing in this bat. The basilar membrane was found to differ from those of other mammals and other bats by showing virtually no change in width except at the extreme ends. Thickenings of the pars pectinata and pars tecta are well developed in Pteronotus; they show no sudden changes in their dimensions and in this way differ from the thickenings found in the European horseshoe bat whose sharply tuned sense of hearing seems at least partially dependent on sudden, marked changes in the structure of the basilar membrane. In Pteronotus the greater part of the basilar membrane, 7.5 mm or approximately 58%, lies within the enormous basal turn and within this turn there are steeply banked curves and one small 0.5-mm region where the membrane is straight. The straight portion is associated with a region of the cochlea where there is a marked change in the density of nerve fibers and where the stria vascularis, spiral ligament and fluid-filled spaces of the ear are enlarged.