A new species of the genus Trichodina, T. dohrni, is described from the gills of a Bathygobius capitoCuv. et Val. from the Gulf of Naples. The new species is characterized by the absence of the inner aboral cilia, the height of the adoral cilia which never is situated higher than the midst of the body. Differences concerning the measures of the denticles are given in a table. The pathogenicity of the parasite is of a high degree, for the gills were covered wich a greyish layer of hundreds of parasites, the respiratory tissues were swollen at many spots.