The gross form changes undergone by trypanosomes when changing their environment are accompanied by adaptive activation and repression of metabolic pathways and correlated ultrastructural changes. In the life cycle of sleeping sickness trypanosomes ( Trypanosome brucei ), the most prominent changes occur in the mitochondrial system in relation to switches in the pathways of energy matabolism, and in the trypanosome surface in relation to evasion of the mammalian host's immune response through antigenic variation. The agent of South American trypanosomiasis ( T. cruzi ), exhibits neither a marked mitochondrial cycle nor antigenic variation, but changes in its surface components in relation to invasion of host cells and resistance to host defence mechanisms have been demonstrated. The study of trypanosome survival mechanisms may suggest ways of halting the development cycle and hence the progress of the disease.