Abstract: In reviewing the earlier works on gill-diseases-of fishes, it is found that there arc two groups of actiological agents. One comprises animal parasites, which inclule Protozoa, Monogenetic Trematoda, Copepoda, Branchiura, etc.; and the other, Phycomycetes (Branchiomycetes), and Schizomyectes (Myxobacteria); but there is, apparently, no authentic record of Eubacteria in the rôle of proven actiological agents in gill-diseases.