1. Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
2. School of Life Sciences, WTB/MSI complex, University of Dundee, Dundee DD1 5EH, UK

Correspondence to: T. Martin Embley¹ Correspondence and requests for materials should be addressed to T.M.E. (e-mail: Email: tme@nhm.ac.uk). The sequence of T. hominis mtHsp70 has been deposited in GenBank under accession code AF492453.

Microsporidia are obligate intracellular parasites of several eukaryotes. They have a highly complex and unique infection apparatus but otherwise appear structurally simple¹. Microsporidia are thought to lack typical eukaryotic organelles, such as mitochondria and peroxisomes. This has been interpreted as support for the hypothesis that these peculiar eukaryotes diverged before the mitochondrial endosymbiosis, which would make them one of the earliest offshoots in eukaryotic evolution^{2, 3}. But microsporidial nuclear genes that encode orthologues of typical mitochondrial heatshock Hsp70 proteins have been detected, which provides evidence for secondary loss of the organelle or endosymbiont^{4, 5, 6}. In addition, gene trees and more sophisticated phylogenetic analyses have recovered microsporidia as the relatives of fungi, rather than as basal eukaryotes^{7, 8, 9}. Here we show that a highly specific antibody raised against a Trachipleistophora hominis Hsp70 protein detects the presence, under light and electron microscopy, of numerous tiny (50 90 nm) organelles with double membranes in this human microsporidial parasite. The finding of relictual mitochondria in microsporidia provides further evidence of the reluctance of eukaryotes to lose the mitochondrial organelle, even when its canonical function of aerobic respiration has been apparently lost.