Bovine syncytial virus (BSV) was isolated by Malmquist, Van Der Maaten & Boothe (1969). Their description of the agent included thin section studies which indicated that the virus has some features in common with leukaemia viruses and the Bittner agent. However, the structure described corresponds closely to that described for foamy agents (Clarke, Attridge & Gay, 1969; Clarke, Gay & Attridge, 1969) and, furthermore, a recent fluorescent antibody study of foamy virus (Fleming & Clarke, 1970) showed a distribution of antigen similar to that found for BSV (Malmquist et al. 1969). In an attempt to clarify the relationship of BSV to foamy virus we obtained a strain of BSV from Dr M. J. Van Der Maaten and examined its structure by negative staining.