SUMMARY The morphology and development of simian haemorrhagic fever virus was studied in tissue cultures infected with low multiplicities of virus. Early cytoplasmic lesions which were observed 24 hr after infection consisted of increased concentra- tions of polysomes and small vesicles IOO nm. in diameter filled with densely staining fibrillar material. As the lesions progressed, thickened membranous structures developed at the periphery of the vesicles. The membranes appeared to lengthen and thicken by a process of end-to-end and side-to-side fusion. Serial sections demonstrated that the fused, thickened structures were lamellae. Their role in the morphogenesis of simian haemorrhagic fever virus is not presently known. Spherical virus particles measuring 4o to 45 nm. in diameter appeared in cells by 72 hr after infection. Infected cells contained vacuoles which enclosed accumula- tions of enveloped spherical particles. Some particles appeared to be budding into vacuoles.