The morphology of in vitro grown lower trypanosomatids L. peterhoffi was studied by means of electron microscopy. The flagellates from both liquid and solid culture media are represented by uninucleate cells of two structural types. Type I flagellates are characterized by dense cytoplasm enriched with numerous ribosomes. Type II flagellates are most abundant in the cultures; they display a less dense cytoplasm and fewer ribosomes. The flagella of L. peterhoffi of both types form enlargements, which are most expressed at the outlet of the flagellar pocket. The nuclei of some cells contain twisted threads about 10 nm in diameter. L. peterhoffi from the liquid media usually possess long, narrow and curved flagellar pockets. On the solid medium, amoeboid and hemispherical colonies composed of both uninucleate and giant multinucleate cells are formed. In these cells the flagellar pockets are usually short and straight. Outside the flagellar pocket, the axoneme often becomes looped in the flagellar enlargements of the colonial uninucleate cells.