Onychomycoses are fungal infections that affect hands and feet nails and are caused by several genera of filamentous and yeast fungi. The knowledge of the etiology is fundamental to establish the appropriate therapeutic conduct. Most fungi genera are free living, but some have adapted to parasitism and others depend on predisposing factors for implantation in the host. The objective of this research was to perform an epidemiological survey to identify the genera of fungi etiological agents of onychomycosis in patients attending in Rio de Janeiro dermatology clinics. After cleaning the nails with 70% alcohol, scraped scales with a sterilized scalpel were collected and seeded in Sabouraud-dextrose-agar and Mycosel culture media at room temperature. The colonies of filamentous fungi were identified by the cultural characteristics and the yeasturiformes by the staining of Gram method and biochemical tests. Among the 1290 cultures examined, 362 (28.06%) were positive for the following fungi: Candida spp. 335 (92.54%), Trichophyton rubrum 12 (3.31%), Epidermophyton floccosum 8 (2.21%), Microsporum canis 6 (1.66%) and Aspergillus niger 1 (0.28%). The fungal etiology of the nail lesions had an incidence of 28.06% of the samples from 1290 patients, being the genus Candida the most prevalent cause of onychomycosis