Tetratrichomonadosis of geese caused by Tetratrichomonas anseris (Hegner 1929; Levine 1973) was diagnosed for the first time in Poland. The disease of a high mortality affected one year old and four years old birds from two reproductive flocks by the end of laying season. The following gross lesions were noted in dead birds: cachexia, enormous ascites, pale-yellow coeca strongly enlarged full the alimentary content. Grey yellow deposits of a hard consistency accumulated in coeca. In microscopic preparates prepared from coeca multiplication of lymphoid cells, accumulation of eosinophiles in mucosa and hypertrophy of muscular layer was observed. A few hours after death trichomonads were not found. Diagnosis was based on the visualization under light microscope (10 X 10, 10 X 40) many motile protozoans classified as T. anseris in swabs prepared from mucous membrane of cloaca. Metronidazole at dose of 10 mg of an active substance/kg bw for 10 consecutive days in drinking water (1 l of water/goose) was used to treat sick birds. After one week breaks metronidazole was applied at a prophylactic dose e.g. 5 mg of an active substance/kg bw in drinking water for 5 consecutive days. Moreover, geese houses were disinfected with 2.0% Pollena JK, shifts were plowed and disinfected with a limewater.