Trypanosomatid parasites cause serious diseases among humans, livestock, and plants. They belong to the order of the Kinetoplastida and form, together with the Euglenida, the phylum Euglenozoa. Euglenoid algae possess plastids capable of photosynthesis, but plastids are unknown in trypanosomatids. Here we present molecular evidence that trypanosomatids possessed a plastid at some point in their evolutionary history. Extant trypanosomatid parasites, such as Trypanosoma and Leishmania, contain several "plant-like" genes encoding homologs of proteins found in either chloroplasts or the cytosol of plants and algae. The data suggest that kinetoplastids and euglenoids acquired plastids by endosymbiosis before their divergence and that the former lineage subsequently lost the organelle but retained numerous genes. Several of the proteins encoded by these genes are now, in the parasites, found inside highly specialized peroxisomes, called glycosomes, absent from all other eukaryotes, including euglenoids.