Dylox® was effective against some fish parasites, ineffective against others. Although experimental results were inconsistent, a few workers report success in treating trichodinid protozoans. No success resulted from treating catfish with Ichthyophthirius. Applications of 0.25 ppm (active ingredient! successfully controlled monogenetic trematodes of Gyrodactylus, Dactylogyrus, and Cleidodiscus spp. Parasitic copepods were effectively controlled by weekly applications of 0.25 ppm except during periods of hot weather. Five species of leeches proved susceptible to single applications of 0.5 ppm Dylox. Factors affecting the decomposition of Dylox in ponds are discussed along with their relevance toward the treatment of ponds. limited data concerning residues following various application rates are presented.