Protothecosis is an infection caused by achlorophyllic algae of the genus Prototheca which rarely affects humans. Some 100 cases have been described in the medical literature, the majority caused by the species P. wickerhamii. The skin is the organ most frequently involved. Diagnosis is performed by isolation of the microorganism in culture or by histopathology. The ideal treatment has not been defined, with amphotericin B and the azoles having been employed. Surgical excision is recommended for small, localized lesions. We describe a case of cutaneous protothecosis on the right fourth finger of a female patient 59 years old with no underlying disease. Administration of itraconazole 400 mg/day for 6 weeks failed to produce an adequate clinical response. Treatment was then changed to fluconazole 200 mg/day, with regression of the lesion.