Summary Bovine syncytial virus was found to replicate in BHK 21 cells. A thin-section study of the morphogenesis of the virus in BHK 21 cells indicated that virus precursor is formed in the nucleus and, in some cells at least, is liberated into the cytoplasm by breakdown of the nuclear envelope. Internal components were found in the nucleus and the cytoplasm and also embedded in the chromosomes of dividing nuclei. Virus internal components gained their envelopes by budding through one of a variety of cytoplasmic membranes, including the plasma membrane. Some stages of the morphogenesis of bovine syncytial virus in BESp cells, and of a type 1 simian foamy virus in BHK 21 cells, are also illustrated. It is concluded that bovine syncytial virus, a feline agent and simian foamy agents all belong to the same virus group.