Abstract The coat protein of tomato bushy stunt virus was investigated by polyacrylamide gel electrophoresis in urea and in sodium dodecyl sulfate (SDS)-containing buffers. The results showed that the virus is constituted of one kind of protein subunit, and it was concluded that the inner and outer shells of this virus result from different spatial arrangement of the same protein. The molecular weight of this subunit was determined by a comparison of its mobility in SDS-polyacrylamide gels to the mobility of protein markers of known molecular weights. The value of 40,000 obtained by this procedure is considerably lower than earlier data in the literature. The amino acid composition of three different strains of this virus is also reported.