A strain of a novel pathogenic, achlorophyllic alga belonging to the genus Prototheca was isolated from the inflamed skin of a patient with protothecosis in a Japanese hospital. The pathogen was detected and isolated in biopsy specimens by histopathology and culture-based examination. Analyses of the nuclear 18S rDNA gene and 26S rDNA gene D1/D2 domain sequences and chemotaxonomic studies indicated that this strain represents a novel species with a close phylogenetic relationship to Prototheca wickerhamii and Auxenochlorella protothecoides. This strain grew well at 28-30 degrees C, showed slow and weak growth at 37 degrees C, and no growth at 40 degrees C. This strain grew in vitamin-free medium and assimilated acetate (pH 5.1), l-arabinose and soluble starch as a carbon source. The taxonomic description of Prototheca cutis sp. nov. is proposed (type strain JCM 15793(T) =CBS 11262(T) =DSM 22084(T)) as a pathogen of dermatitis.