Three proteins with nominal molecular weights of 73 kDa (XNF-L), 175 kDa (XNF-M), and 205 kDa (XNF-H) were identified as putative neurofilament proteins in the nervous system of the frog, Xenopus laevis. These conclusions were based on four criteria: (1) these proteins were enriched in cytoskeletal preparations; (2) they reacted with a monoclonal antibody (anti-IFA) that cross-reacts with an epitope found in all intermediate filament proteins; (3) they cross-reacted with monoclonal antibodies directed against specific mammalian neurofilaments; and (4) antibodies that reacted with these proteins on Western blots specifically stained neurons in immunohistochemical analyses. The neurofilament proteins in Xenopus were antigenically similar, but not identical to mammalian neurofilament proteins. The principal difference was that four antibodies that reacted on Western blots with rat NF-H reacted with XNF-M in Xenopus. However, similarly to mammals, antibodies against phosphorylated XNF-M specifically labeled axons, whereas an antibody that reacted only with dephosphorylated epitopes on XNF-M specifically labeled neuronal cell bodies in immunohistochemistry. Three other antibodies that reacted equally well with untreated or alkaline-phosphatase-treated XNF-M or XNF-H proteins also showed axonally restricted staining in the adult Xenopus nervous system. An XNF-L (XC5D10) antibody was produced which stained axons and cell bodies equivalently throughout the adult Xenopus nervous system. By 3 days of development (stage 42; Xenopus tadpoles), antibodies to all three molecular weight forms of the frog neurofilament proteins detected specific neurons in the brainstem and spinal cord; and antibodies to phosphorylated and dephosphorylated epitopes on XNF-M could discriminate between axons and cell bodies in the rhombencephalon. The immunoreactivities of four antibodies directed at XNF-L, -M, or -H, which were unaffected by alkaline phosphatase treatment, differed significantly in their immunohistochemical staining patterns in adult vs. premetamorphic frogs.