A patient had cutaneous protothecosis because of the alga-like organism, Prototheca wickerhamii. In vitro sensitivity tests showed that the organism was sensitive to amphotericin B, and was treated successfully with this polyene antibiotic. As with treatment of some fungal infections, a clinical response was achieved when therapy with low doses of amphotericin B was given during a short period of time. The basis of the amphotericin B response may have been due to a combination of its immunostimulatory and antibiotic properties.