Monogenean parasites are usually well accommodated to their fish hosts in nature; however, they frequently cause severe epizootics in cultured or aquarium held populations. The short, direct, one-host life cycles of monogeneans enable them to reach epizootic levels quickly when hosts and parasites are confined closely together. Further, debilitating attacks by these helminths may be accompanied by invasions of viruses, bacteria, and fungi. Monogeneans of the superfamily Microcotyloidea cause mortality in aquaculture and public aquariums. They attach to gill filaments within the protected habitat of the opercular cavity making them much harder to dislodge than other monogeneans that infest skin. Thus far, attempts to eliminate infestations of microcotyloidean monogeneans in culture have not been very successful. Infestations of Microcotyle hiatuiae on the gills of tautog, Tautoga onita provided a model for testing the effects of praziquantel, mebendazole, flubendazole, formalin, and copper sulphate on adult monogeneans, oncomiracidia, and eggs. Of the chemicals tested in vitro, praziquantel and formalin were the most effective against adult worms. Eggs are extremely resistant; however, long term treatment with copper sulphate was found to be the most effective. Sanaqua and calcium hypochloride were also tested as disinfectants. Results from several other monogenean species also are compared.