Abstract Lobomycosis is a skin infection produced by the fungus Lacazia loboi , which mainly affects some indigenous and afro‐descendant populations in Tropical America. We previously reported the comparative effect of osmium tetroxide (OsO 4 ) and ruthenium tetroxide (RuO 4 ) in the electron microscopy (EM) of other related microorganisms. The objective of this study is to compare the effect of postfixation with OsO 4 and RuO 4 in the ultrastructure of L . loboi yeasts. Skin biopsies on patients diagnosed with lobomycosis were fixed in glutaraldehyde at 3% and postfixed in the following solutions: (a) 1% OsO 4 , (b) 0.2% RuO 4 , and (c) OsO 4 at 1% followed by RuO 4 at 0.2%. They were then processed using the conventional method for EM. Unlike OsO 4, the treatment with RuO 4 revealed different shades of gray and electron dense bands in the cell wall and other cell components of L . loboi . The most notable finding was the presence of radial filamentous structures around the yeast, which made the image look like the sun. Postfixation with RuO 4 revealed ultrastructural details that had not been previously reported for L loboi . The combined use of OsO 4 and RuO 4 in EM of microorganisms with cell walls can be useful to evaluate the effect of microbicide substances.