We present a 45 year-old man with an eight-year history of discoloration of the nail plate on his left hallux. He had been treated with two courses of oral terbinafine and topical 8% ciclopirox for presumed onychomycosis. On exam, his left great toenail contained a wide yellow-white longitudinal band involving a majority of the nail plate. No subungual debris, hyperkeratosis, or paronychial inflammation was present in the affected nail. Histopathology of the nail plate revealed numerous fungal elements arranged transversely and longitudinally, solely within the keratin layers of the nail plate; these were highlighted with periodic acid-Schiff (PAS) stain confirming endonyx onychomycosis. Cultures grew Trichophyton rubrum. All types of onychomycosis under the new classification system proposed by Hay et al. have now been associated with T. rubrum. Endonyx related to T. rubrum may be a particularly difficult infection to treat with oral or topical agents owing to the absence of robust local immune response and limited drug penetration to the interior nail plate. Physicians should be aware that this type of infection may require treatment with dual-agent therapy or alternative modalities including chemical or surgical plate avulsion or photodynamic therapy.