Conventional transmission electron microscopy was used to localise double-membrane vesicles probably representing mitochondrial remnants ("mitosomes") in four species of microsporidia. Very few such vesicles were found dispersed throughout cytoplasm with no relationship to other cell organelles. Several double-membrane vesicles per ultrathin section, however, occurred regularly close to the nuclear spindle plaque. These vesicles are identical with the "polar vesicles" typically associated with the microsporidian spindle plaque and known since 1971. The reason for mitosome accumulation near the spindle plaque is unknown. Possibly the spindle plaques are involved in mitosome segregation during cell division.