The gut protozoan parasite, Giardia duodenalis, is the best characterized example of the most ancient eukaryotes, which are anaerobic and appear to be primitively amitochondrial. Apart from its obvious medical importance, Giardia is fascinating in its own right. Its prokaryotic-like anaerobic metabolism renders it selectively sensitive to some bacterial drugs, especially the nitroimidazoles, which are activated to form toxic radicals. Other features, including an enzyme that reduces oxygen directly to water, cysteine as the keeper of redox balance, a plasmid, and toxin-like genes are also distinctly prokaryotic-like. But, unlike prokaryotes, Giardia has a sophisticated, highly developed cytoskeleton, bounded nuclei, linear chromosomes capped with telomeric repeats, and telomere positional regulation of gene expression.