Madurella mycetomi, the fungus pathogen of black grain mycetoma, shows an elaborate development of mesosomes as its most striking ultrastructural feature. These occur in addition to mitochondria and endoplasmic reticulum. Condensations of hyphal cytoplasm and a recurring intrahyphal growth of new hyphal cells are features of the pigment-producing and pathogenic phases of the organism. The abundant pigment which histochemically resembled melanin was not traceable to any ultrastructural granule, nor was it built from a tyrosine precursor. The ultrastructural changes occurring in collagen through a sclerotizing action by the fungus are briefly noted.