Abstract: According to the present knowledge,gill diseases of the grass carp (Ctenopharyn-godon idellus) can be broadly arranged under three categories,viz.,diseases causedby parasites,by branchiomycosis,and by bacterial infections.Among these,bacterialinfections are the most serious menace to grass carp culture and cause much of themortality among both fingedlings and yearlings.Unfortunately,gill disease has hith-erto received little attention since there has been a prevailing belief that the highmortality is due to intestinal infections.Casual attempts to isolate the pathogen ofgill-rot were unsuccessful either in our Institute or in other laboratories of this coun-try.As a result of this situation,no effective measures were adopted to halt thecourse of the infection and this rendered many hatcheries to sustain considerable mortalities year after year.In the late summer of 1972,we restarted the isolation of this particular agentduring the outbreak of an epizootic in grass carp fingerlings at the East West Lake Hatchery and our fish ponds in Wuban,Hupei Province.Experiments on infectionand transmission were carried out,and the disease commonly referred to as gill-rotwas reproduced in the healthy fish.Pure cultures of the etiological agent werereisolated from the experimentally infected fish,thus fulfilling the requirements of the Koch's Law.Of the 19 isolated strains which were found to be more or less pathogenic to thegrass carp when experimentally tested,strain G4 was the most virulent,so we madechoice of this particular strain for a more thorough study.Emphasis has been placedon its morphology,life cycle including the fruiting body and microcysts,and the for-mation of columns.The physiological and biochemical properties were also studied.On the basis of the characteristics of the fruiting body,the present species shouldbe assigned to the family Myxococcaceae of the order Myxobacterales.According tothe key to the genera of the family Myxococcaceae proposed by Stanier (1957),thespecies should be placed in the genus Myxococcus,since the fruiting bodies on agarare conical in shape,being surrounded by a thin membrane without any peduncle orbranches and are easily deliquescent.There are now at least six species listed in the genus Myxococcus (Stanier,1957).The present species somewhat resembles M.xanthus Beebe (1941) in color and size ofthe fruiting body,but differs from the latter in the shape and size of the vegetativecell and in the lacking of constriction at the base of the fruitng body,as well as inthe habitat.The present species is an animal parasite,whereas the six known speciesare all found on decaying organic matter in soil or in the dung of various animals.On the ground of these differences,it seems justified to regard it as a new species,for which the name Myxococcus piscicola,Lu,Nie Ko,sp.nov.is proposed.In connection with the disputes in the literature over the nomenclature and taxo-nomic position of the well-known etiological agent of“columnaris disease”,thefollowing comment may be made.Ordal and Rucker (1944) first isolated an etiolo-gical agent in pure culture from the surface lesions of sockeye salmon fingerlings atthe U.S.Fish and Wildlife Service hatchery at Leavenworth,Washington,andidentified it as Chondrococcus columnaris on account of its“columnar or branchingfruiting bodies”.In reality,the so-called columnar or branching fruiting bodies arenot true fruiting bodies at all,because they,in our experience,could never transforminto spherical or oval microcysts,and were finally to degenerate into spheroblasts.Ordal and Bucker did isolate strains with definite fruiting bodies from seven Columbia River fishes,but these strains,as they pointed out,exhibited some differences in growthon solid media as compared with the strain of Leavenworth.It is to be regretted thatthey took it for granted that the strain of Leavenworth would form the same kind offruiting bodies as the Columbian strains do.Inasmuch as the former is a non-fruiting-body-bearing form and thus should be placed in the genus Cytophaga,Garn-jobst's (1945) identification as Cytophaga columnaris seems to be correct.