The distribution of filamentous actin (F-actin) in outer hair cells has been examined in several mammalian species using tetramethylrhodamine phalloidin, a specific marker for F-actin. The stereocilia and cuticular plates of the OHC in all species examined (pigmented guinea pig, hooded rat, chinchilla and squirrel monkey) contained F-actin; however, an infracuticular network of F-actin was present in OHC of the apical turns of the guinea pig cochlea but could not be identified in any other species examined.