ABSTRACTThe ultrastructure ofTrypanosoma brucei gambiensewas investigated by the freeze‐fracture method. Three different regions of the continuous plasma membrane; cell body proper, flagellar pocket, and flagellum were compared in density and distribution of the intramembranous particles (IMP's). The IMP‐density was highest in the flagellar pocket membrane and lowest in flagellum. Intra membranous particles of the cell body membrane were distributed uniformly on both the protoplasmic (P) and exoplasmic (E) faces. On the P face of the flagellar membrane, a single row of IMP‐clusters was seen along the juncture of the flagllum to the cell body. Since the spacing of the IMP‐clusters was almost equal to the spacing of the paired rivet structures observed in thin section, these clusters likely are related to the junction of flagellum and cell body. At the neck of the flagellar pocket, several linear arrays of IMP's were found on the P face of the flagellar membrane, while on the E face rows of depressions were seen. At the flagellar base, the clusters of IMP's were only seen on the P face. On the flagellar pocket membrane, particle‐rich depressions and linear particle arrays were also found on the P face, while on the E face such special particle arrangements were not recognized. These particle‐rich depressions may correspond to the sites of pinocytosis of the bloodstream forms which have been demonstrated in th