SUMMARYRift Valley fever virus isolates from the 1977 outbreak in Egypt were studied at an ultrastructural level. The particles measured 90 to 11o nm in diam. using negative staining and sectioning techniques, with a core component of 8o to 85 nm. The surface of the virions was calculated to be covered by approx. 16o sub-units. The particles were found in smooth endoplasmic reticular systems, which were made up of either multi-tubular complexes, or of a single large vacuole. The majority of these membrane systems were found to be unassociated with Golgi apparatus. Inclusion bodies were found within the host cell nuclei (made up of rods and fine granules) and in the cytoplasm (aggregates of fine or coarse granules). The possible relationship of these structures to virus replication is discussed.

The course of Trypanosoma congolense infections in Glossina morsitans morsitans was followed by electron-microscopic examination of ultrathin sections of the guts and proboscises of infected flies. Guts dissected from flies 7 days after infection with culture procyclic forms of T. congolense had heavy trypanosome infections in the midgut involving both the endo- and ectoperitrophic spaces. Trypanosomes were also seen in the process of penetrating the fully formed peritrophic membrane in the central region of the midgut. By post infection day 21, trypanosomes had reached the proboscis of the fly and were found as clumps of epimastigote forms attached to the labrum by hemidesmosomes between their flagella and the chitinous lining of the food canal. Desmosome connections were observed between the flagella of adjacent epimastigotes. Flies examined at postinfection days 28 and 42 had, in addition to the attached forms in the labrum, free forms in the hypopharynx.

Ultrastructural studies of the mechanism of penetration of the salivary gland of the reduviid bug Rhodnius prolixus by Trypanosoma rangeli showed that trypanosomes from the haemocoele penetrate the outer "membranes" of the gland flagellum foremost, disrupting the inner layers, to pass between the muscle cells to reach the gland cell basement membrane. This latter is also penetrated flagellum foremost, the parasite invaginating the gland cell plasmalemma beneath, to create a vacuole in which the trypanosome crosses the gland cells to reach the central lumen, often only losing its containing vacuole just before leaving the cell. The structure of the outer "membranes" surrounding the salivary gland appearland cells. These outer "membranes" were found to enclose large numbers of multinucleate "giant form" trypanosomes, whose significance is as yet unknown, but could perhaps represent a stage in the life cycle of the parasite where genetic interchange could take place.