Additional information on host interactions with trypanosomatid membranes was obtained from studies of a monomorphic strain of Trypanosoma brucei harvested at peak parasitemia from intact and lethally irradiated rats. Pellets of trypanosomes were fixed briefly in glutaraldehyde and processed for thin section electron microscopy or freeze-cleave replicas. Observations of sectioned material facilitated orientation and comparison of details seen in replicas. Fracture faces of cell body and flagellar membranes as well as 3-dimensional views of the nuclear membrane were studied. Cell body membranes of 80% of the organisms from intact rats contained random arrays of intramembranous particles (IMP). Aggregated clusters of particles appeared on the fracture faces of 20% of the trypanosomes. Some of these membranes had nonrandomly distributed particles aligned in distinct rows on the outer fracture face of both cell body and flagellum. Many inner face fractures of the cell body membranes had a particle arrangement similar to the longitudinal alignment of cytoskeletal microtubules. No aggregated particle distribution was seen in membranes of trypanosomes harvested from lethally irradiated rats. Replicas of trypanosome pellets also had plasmanemes as a series of attached, empty, coated membrane vesicles. These structures were found in close association with, as well as widely separated from the parasites. The shedding of these vesicles and the variation of particles in cell body membranes are discussed in light of antibody-induced architectural and antigenic changes in surface properties of trypanosomatids. The convex face of the inner membrane of the nucleus also is covered with randomly arrayed particles. More IMP were observed on the inner than on the outer nuclear membranes. Images of nuclear pores were also seen. The importance of these structures in drug and developmental studies of trypanosomes is discussed. On fracture faces of the flagellar membrane there were miniature maculae adherentes, unique to the inner fracture face and occurring only at regions of membrane apposition between cell body and flagellum. Each cluster of particles exposed by the freeze-cleave method corresponds to an electron-dense plaque seen in thin section images. However, because of a unique fracture pattern, these plaques were not revealed on the apposing body membranes, as illustrated in thin sectioned organisms.