Detailed morphology and life history of Trichodina reticulata, isolated from the mucous of body surface, fins and gills of goldfish and other carps of Japan, were studied.<BR>Generally dome-shaped body measures from 50-90μ in diameter and height of the body is 15-35μ. The diameter of sucking disc is 44-80μ and that of the denticulate ring 42-72μ and the horse-shoe shaped macronucleus 22.5-49μ, the thickness being 6-7μ. Number of denticle is most often 24 and 27(20-31) and radial pins per denticle is 10(7-10). Dimensions of denticle: length of blade 4.5-7.5μ; length of ray 5.5-7.5μ; width of central conical part 2.5-4μ and length of denticle 5-11μ. Width of border membrane 3.5-6μ. Large spindle-shaped micronucleus (7-15×2-5μ) lies in a shallow depression on the outside of right arm of the macronucleus. The central part of the sucking disc is separated into several fields of cell-like structures. Both contractile and food vacuoles are distinct. Velum is delicate and transparent. The ciliate moves by means of velum, aboral cilia and the membranelles. Both ciliary girdles lie in usual manner. Adoral zone forms a spiral of about 370°.<BR>Binary fission starts with the enlargement of the ciliate, changes in shpae of the macronucleus, becoming short and thick with randomly distributed chromatin; finally dividing into two dumbell-shaped equal halves. The micronucleus swells and divides mitotically. Prior to the onset of fission the anlage of new denticulate ring appears between the old ring and circumference of the disc. Indentations appear in the velum and breaks appear in the striated band below the velum. Contractile vacuole divides to produce one for each daughter cell. After division the old ring with half the number of denticles gradually disappears and about twice the number of denticles are developed in the new outer ring. Similarly radial pins and cilia also restore original number.<BR>A few cases of conjugation were found where both iso and anisogamous types were observed. The large exconjugant contains one functional nucleus, the synkaryon, which divides three times and thus forming eight micronuclei, seven of them micronuclear anlagen and one functional micronucleus. The cell division continues until each of the daughter individuals contain one macronuclear anlage and a micronucleus. In this case the number of denticles in the new ring of the daughters is the same as in the old ring