The sacculi and lagenae from representatives of five species of catfishes were studied by light microscopy to provide an account of the anatomy of those structures and to determine whether phylogenetic changes can be detected in the inferior division of the membranous labyrinth among closely related ostariophysian fishes. The lagena showed little change among the forms used, while the saccule exhibited marked interspecies modifications. Anatomical variation was evident in the shape of the saccule, positioning of the interconnecting transverse canal, presence or absence of a release membrane, and, more notably, the position of the saccular macula and the shape and fluting pattern of the sagitta. Only minor variations were evident in the saccule and sagitta between the "primitive" conditions displayed in the siluroid form and those in the two bagroid representatives; in the latter forms, however, some positional modification of the posterior part of the macula from the "primitive" arrangement was present. In the bagroid representatives, the general morphology appeared to be stabilized early in the radiation, and individual changes suggest two lines of development within the group. The most significant modifications were evident in the loricaroid group where progression from the early representative to the more advanced form showed major changes in the inferior division of the labyrinth, including loss of the anterior part of the saccule and sagitta, modification of the remaining part of the sagitta, and repositioning of the saccular macula. The pattern changes in the saccule generally agreed with the phylogenetic sequence proposed by Chardon ('68) for catfishes.
