When epimastigote forms of Trypanosoma cruzi grown in a rich medium (LIT) are transferred to a simple, chemically defined medium (TAU3AAG, containing Ca2+, Mg2+, K+, Na+, L-proline, L-glutamate, and L-aspartate in phosphate buffer) they transform into trypomastigote forms. Morphometric analysis of transmission electron micrographs of thin sections of parasites collected at different steps of the transformation process showed that no changes occurred in the volume density of mitochondria and cytoplasmic vacuoles. However, a significant increase in the volume density of the kinetoplast DNA network as well as the lipid inclusions and a decrease in that of the reservosome (a special type of endosome) was observed. These observations suggest that during differentiation, T. cruzi accumulates lipids and uses molecules contained in the reservosome as its main energy source.