Protozoans are interesting creatures for several reasons, not least of which is their ability to live inside or among the cells of their hos t while resisting the host's cellular defences. The rigours of their l ifestyle have resulted in the evolution of some remarkable adaptation at the biochemical level; many parasites have dispensed with entire me tabolic pathways, and have evolved complex mechanisms for transmission to new hosts. Parasitic protozoa cause some of the major infectious d iseases of humans and domestic animals. In recent years, new molecular biological techniques have opened...