Abstract: As a species of the epizootic ciliate, A piosoma piscicola Blanchard, 1885 which attaches to the surfaces of many kinds of hydrobionts can badly endanger the hosts and causegreat losses tothe aquaculture industry. It also has awide spread, such as Europe, Asia and South Africa. Because of its great variability in different regions, A piosoma piscicola (Blanchard, 1885) has some synonyms such as/Glossatella cyprinid Lrmek HuÍek, 19530,/A piosoma magna Banina, 19680, etc. In China, it was firstly reported by Chen with the name/G lossatella cylindriformis0 in 1956. Although Apiosoma piscicola was found more than one century before, today our understanding to them remains in the stage of alpha taxonomy. In order to carry out the research deeply, the author collected numerous specimens from the gills and fins of the fishes that were heavily infected in the Hongze Lake of Jiangsu Province located in east of China. This paper systematically dealt with themorphology and structure of the living specimens(stained with 2% Neutral Red)and fixed-stained specimens(the smears fixed in saturated HgCl2solution and stained with Heidenhain. s haematoxylin, Ehrlich.s haematoxylin and the dry smears impregnated in 3% AgNO3solution)under optical microscopy and the ultra structure under transmission electron microscopy. Great diversities have been found on the sizes of the organisms (the length and width of the body, macronucleus and micronucleus)compared with the data ofHupei specimens and the reason was discussed. In addition, lots of bacteria and food particles were observed within the body of the organisms, especially around the peristomial discunder transmission electron microscopy. This phenomenon gives a strong support fromthe aspect aof food sources to the viewpoint that Apiosoma is a kind of ectocommensals but not ectoparasites.