Based on the general cross-reactivity of the cytokeratins in vertebrates, we describe the immunoreactivity for some mammalian cytokeratins of both the epidermis and gill of H. fossilis. The following monoclonal antibodies, commercially supplied, were tested: K8.13, KL1, AE1 and AE3, which have a wide spectrum of specificity, and LDS-68, M 20, K8.60, KS-B17.2, K4.62, which are more narrowly specific. The reaction of the epithelial cells of the skin to K8.13 was negative in the basal layer, weakly positive in the layers above but strongly positive in some cells of the superficial layer. KL1 was negative in the basal layer, positive in the outer layers. AE1 was strongly positive in the basal layer, negative in the superficial cells. AE3 gave a general but weak reaction in the epithelial cells. K8.60 was negative for the epithelial cells, but reacted positively in the club cells. Club cells also reacted to K8.13 and AE1, and were strongly positive to KL1 and AE3. Goblet mucous cells were negative to all antibodies tested. In the gills, K8.13 labelled cells of both filament and lamellar epithelia. The reaction to AE3 was similar but less intense. KL1 was positive in the basal layer of the filament epithelium but not elsewhere, and K8.60 was negative throughout. AE1 and KS- B17.2 specifically labelled small cells scattered in the filament and lamellar epithelia, which are tentatively identified as neuroendocrine cells.