The trypomastigote stage of Trypanosoma lewisi swims in the blood of rats.
This photo shows three trypomastigotes. there are numerous erythrocytes in the background, and a single host leucocyte (in this case a polymorphonuclear leucocyte, or neutrophil) in the top left. The trypomastigote body shape is undulating, characteristic of many species of trypanosome. The nucleus is large and is located in the centre of the trypomastigote; it is stained darkly only in the specimen at the lower right. The smaller, densely-staining kinetoplast is midway between the nucleus and the end of the body. The flagellum is visible in all specimens and the undulating membrane is clearly visible in four.