A new species, Blastocystis galli, parasitic in blind processes of large intestine was found in domestic hens. Sizes of blastocysts are 7.5-35.0 x 6.25-30.0 (18.67 x 17.05) microns. The parasite form varies from round to ellipsoid. There were found stages with 1 to 4 nuclei and stages containing 8 to 32 small daughter individuals. Outside blastocysts are covered with structured glycocalyx. Under glycocalyx there is a plasmatic membrane. Cytoplasm contains a great number of ribosomes and mitochondria with cristae resembling in their shape oval or round small sacs. Nucleus contains nucleolus. Chromatin mass is concentrated on one of the poles of the nucleus as individual bodies. Semilunar in form chromatin mass was not found. Golgi apparatus is represented by a number of plates grouped in a pile. Most part of the cell is occupied by reproductive organelles divided by cytoplasmatic membranes into compartments. On the basis of its ultrafine organization. B. galli is assigned to the kingdom Protista, type Rhizopoda, class Lobosea, subclass Gymnamoebia, order Blastocystida.