An intramitochondrial fibrous component (IMF) was consistently detected by electron microscopy in all eight strains of yeast examined, either respiratory competent or deficient, and including two species, Saccharomyces cerevisiae and S. uvarum. IMF was always found assembled into a layer 20 nm in thickness. In respiratory-deficient mutants, multiple IMF layers roll up concentrically to give rise to cylindrical inclusion bodies which attain several micrometers in length. In normal respiring yeasts, IMF occurs in close association with cristae to elaborate composite structures in which multiple parallel IMF layers are sandwiched between a pair of cristae. Evidence is presented to demonstrate the extramitochondrial origin of IMF.