Proliferation of saccular hair cells of the amphibian, Xenopus laevis, was examined at various stages of development. Numbers of total hair cells and of hair cell bundle types were determined in larval (stages 47, 52 and 56), recently metamorphosed juvenile (1 g), and adult (60 g) animals with scanning electron microscopy (SEM). Hair cells were identified by ultrastructural analysis of the stereociliary bundle. Two general bundle types were present on the sensory epithelium: long stereociliary (LS) and short stereociliary (SS) bundles. Based on the kinocilium length, the SS bundle type was further divided into SS1 (kinocilium > or = 8 microns) and SS2 (kinocilium < 8 microns). The sensory epithelium was composed of a central zone containing all LS and some SS bundles, and a peripheral zone containing only SS bundles. Our results show that in X. laevis, the number of LS and SS bundles, as well as the ratio of LS/SS bundles increased continuously during larval and post-metamorphic development, with an associated enlargement in the sensory epithelium area. These increases were more pronounced during larval life. The percentage of hair cells with SS1 bundles was greater in larval stages, while that of hair cells with SS2 bundles was comparatively higher in juvenile and adult specimens.