The ranunculaceous derivative protoanemonin (PrA) was studied as an antifungal agent on the dermatophyte Microsporum cookei. The ultrastructural changes that PrA brought about in this fungus were observed with both the transmission and scanning electron microscopes. The main anomalies noted were abnormally shaped hyphae and within the cytoplasm, multimembranous bodies which were irregular in shape and size, and tubules of 25 and 60 nm in diameters. Mitochondria, nuclei and vacuoles were also variously affected by PrA. Although multifarious, the observed cellular alterations in M. cookei can be considered the result of a PrA interaction with cytoplasmic microtubules. Since these cell structures contain a great number of ASH groups, our previous hypothesis, that sulphydryl groups are the primary targets of this molecule, appears to be supported.