The medulla of the unconstricted notochords of the shortnose sturgeon, Acipenser brevirostratus, and African lungfish, Protopterus annectens, and the cellular component of the intervertebral joint tissue of the teleost fish, Perca flavescens, are comprised of cells with a large central vacuole. Previous studies on the fine structure of this tissue revealed that the cytoplasm surrounding these vacuoles consists of 10-nm-diameter intermediate filaments. Since in mammals there are a large number of tissue-specific types of intermediate filaments, this study uses antibodies to mammalian intermediate filaments to determine the type of filaments present in the notochord cells of bony fishes. Positive labeling using a polyclonal antibody to human skin keratins is observed in the cytoplasm of the notochord cells in the intervertebral tissues of Perca. These tissues are also probed with the AE series antibodies that label keratins found in mammalian epithelial cells. In both Protopterus and Acipenser the peripheral cytoplasm of the notochord cells is labeled with all three AE antibodies. In Perca only the AE3 antibody probe produces positive staining. These staining patterns are consistent with previous studies on the localization of cytokeratins in fish tissues and indicate that the intermediate filaments in the notochord cells of bony fishes are immunologically similar to the mammalian keratins.