Protothecosis is an unusual cutaneous soft tissue infection ca by the Prototheca, which is a genus of the unicelluar, achloric algae. We report a case of cutaneous protothecosis in a 66-year-old female, who showed erythematous, purulent patches and plaques with ulcerations on the right forearm for 2 months. Biopsy specimen revealed the characteristic thick-walled morulalike sporangia in the dermis. Prototheca wickerhamii was isolated in the culture and the biochemical study. Electron microscopic examination showed the thick-walled spores containing dark dense bodies and amyloplasts, After two months of oral itraconazole 200mg/day, skin lesions were improved. (Korean J Dermatol 2002;40(9): 1116-1120)