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Background: Little is known about the ultrastructure of Piedraia hortae., Objective: To examine a P. hortae colony with scanning electron microscopy and investigate possible contributions to its the pathogenesis of black piedra., Results: On low magnifications, two distinct aspects of the colony are identified, a compact area and a filamentous area. Analysis of the filamentous area demonstrates hyphae adhered by a thin reticular substance. A recurring finding is the adhesion between the fungal filaments in parallel. On high magnifications, the microfibrillar substance adhering the hyphae to each other becomes very evident. Examination of the compact area shows the hyphae embedded in the reticular matrix forming a biofilm and the colony well adhered. On high magnification, it can be observed that the hyphae are within this fibrillar matrix, which has the same appearance as the filamentous substance that adheres the hyphae to each other., Study Limitations: Only one strain was examined., Conclusions: The formation of biofilm with fungal structures and reticulated extracellular substance is important in the pathogenesis of black piedra., (Copyright © 2024 Sociedade Brasileira de Dermatologia. Published by Elsevier España, S.L.U. All rights reserved.)