The structure of the intracellular form of Leishmania donovani as seen by the light microscope consists of a nucleus and usually a rod-like kinetoplast (Wenyon, 1926) within a homogeneous mass of cytoplasm, while the extracellular or culture form has, in addition to these structures, an anterior flagellum. The intracellular form is usually round or ovoid measuring 2-4 microns; the extracellular form measures about 14-20 microns in length and from 1.5-3.5 microns in breadth. Because of the small dimensions of the protozoan parasite and the limited resolution of the light microscope, relatively little information is available on its fundamental organization. With the great resolving power of the electron microscope, several workers have attempted to study its ultrastructure with this instrument. The studies of Emmel, Jakob and Golz (1942), Sen Gupta et al. (1951), Das Gupta et al. (1954) of this species, and Lofgren (1950) of Leishmania tropica using electron microscopy of the culture forms have failed to yield new information on the fine internal morphology because the parasites were examined as whole specimens. The thickness of the specimens does not allow for adequate electron beam penetration, and the parasites appear as dense masses. In this study however, this problem is eliminated by the use of thin sectioning. Fine structural detail is observed in these preparations in both the extraand intracellular forms. The accumulation of structural knowledge may enable us to understand the relationship of host cells and leishmania; i.e., the biology of this intracellular parasitism. This paper presents only a few observations of this host-parasite relationship and further work is necessary to make a thorough study of this problem.