A 21-day subculture ofRhodosporidium toruloides Banno produced septate mycelium with hook-shaped cells over many septa. In the light microscope these hook-shaped cells appeared to be clamp connections; this feature might suggest thatR. toruloides belongs to the basidiomycetes. Electron micrographs, however, demonstrated that the hyphal septum above which the hook cell occurred and the back septum of the hook cell were tapered centripetally, contained a pore in the middle, and were typical of the ascomycete septum. Two electropaque bodies, believed to be similar to Woronin bodies, were observed occluding the pore in one hyphal septum above which was a hook cell. Melanospores in the 21-day subculture were produced terminally on hyphae and laterally above hook cells which occurred over ascomycete-type septa. Proliferating hook cells appeared similar to repeating croziers found in the ascomycetes. The walls of hyphal cells from the 21-day subculture demonstrated lenticular bodies and alternating electrolucent and electropaque lamellae. The walls of 12-hr dividing yeast cells were observed to be similarly lamellate, a characteristic which has been reported typical of heterobasidiomycetous yeasts. SinceR. toruloides exhibited ascomycetous and basidiomycetous features, a reconsideration of its present classification is suggested.