The fine structure of Urceolaria kozloffi Bradbury, 1970 (fam. Urceolariidae), an ectocommensal of brachiopods Hemithyris psittacea has been investigated with the use of transmission (TEM) and scanning (SEM) electron microscopy. Compared with literary data on the structure of ciliates of the family Trichodinidae, the result of reseach has shown the absence of principal differences in the organization of urceolariids and trichodinids. The body of U. kozloffi is barrel-shaped (80-50 mkm). Adorai spiral of cilia which consists of polykinets and haplokinets, describe a circuit of 400°. The aboral ciliature consists of three ciliated girdles: the upper, the middle and the lower. The kinetosomes of the upper girdle are organized in one row. They produce massive long marginal cirri which are directed upward and orientated in parallel to the body surface. The middle girdle is situated below the marginal cirri. Its kinetosomes form 9 rows and arranged in a chess-board fashion. The lower girdle is situated directly above the border membrane. Its kinetosomes form one row. The girdles are separated from each other by septums. Between the lower and the middle girdles a short and thick septum is located. Its alveolar layer is wellpronounced. No supporting structures of microtubules organized in rows have been observed. Between the middle and the upper girdles a larger septum is situated, below the alveolar layer of which a row of microtubules is located. Well-discernable filamentous roots leave the kinetosomes in the middle and the lower girdles. They are connected with the skeletal elements of the sucker. The adhesive disk is fastened with the skeletal elements of the cytoplasm: with a ring of denticles and radial stripes. The number of denticles in the ring varies from 12 to 14 (n = 50). The denticles are thin, smooth, 5-10 mkm long and 2 mkm wide. The rings diametre is 14-17 mkm. There are about 7 radial stripes (6-10) for each denticle, forming the radially lineated sucker's periphery. Radial stripes are connected with more delicate and numerous pins of the border membrane which forms a border between the adhesive disk and trochal ciliature. Macronucleus is H-shaped, micronucleus is oval and lies in the vicinity of one of the macronucleus branches. U. kozloffi occurs on the tentacles of the lophophore of H. psittacea brachiopods. The member of urceolariids makes up to several hundred of specimens. 112 brachiopods were dissected; out of this number 61 were infected and the rest 51 were not. Correlative analysis between the brachiopods' size and their infection rate has shown (r = 0.5475, p < 0.05) that the older the host the more probable his infection by urceolariids is. Experiments on jount keeping of sterile and infected brachiopods has demonstrated the absence of effective transmission mechanism of ciliate from one host to the other.