A double membrane surrounds the hydrogenosomes of the anaerobic fungus Neocallimastix frontalis.

The structure of hydrogenosomes of the anaerobic fungus Neocallimastix frontalis was analyzed using routine preparations for transmission electron microscopy, freeze-fracture and immunocytochemistry. They appeared as round or elongated structures, always enveloped by two distinct, but tightly apposed membranes. Images of organelle division were very similar to those observed in trichomonad protozoa. These observations suggest that hydrogenosomes are homologous organelles in unrelated species weakening the hypothesis of a polyphyletic origin and supporting the evidence that fungal hydrogenosomes are probably derived from an endosymbiont relationship.