Cases of onychomycosis represent up to 30% of diagnosed superficial fungal infections and are caused hy three groups of fungi: dermatophytes, yeasts, and nondermatophytic molds. The majority of toenail infections are caused hy dermatophytes; Trichophyton rubrum is isolated with the greatest frequency. In infections of fingernails, Candida species can be isolated as frequently as the dermatophytes. Of the molds, Scytalidium species can infect both fingernails and toenails, as well as adjacent skin, and represent 3% of the nail infections in a temperate country such as the United Kingdom but a much higher proportion in tropical countries. Other molds such as Scopulariopsis, Acremonium, and Aspergillus species can infect damaged nails. The isolation of a dermatophyte is always considered indicative of infection, but the presence of other molds, which may be aerial contaminants, must be interpreted with care.