Protothecosis is a rare infection caused by algae of the Prototheca genus. Its diagnosis is based on microbiological studies and on the identification in biopsies of the characteristic structures of the algae. We present the histopathological findings of protothecosis in a 72 year-old woman with the presumptive diagnosis of chronic actinic dermatitis. The pathological study of a skin biopsy revealed structures with internal septa typical of Prototheca. A review of all reports on this disease in Colombia is also presented.