Quantitative electron microscopy and polyacrylamide gel electrophoresis have been applied to characterize and compare the structures of three viruses of prokaryotes. The molecular weights of blue-green algal viruses LPP-1M and LPP-2 and phage T7 are (in millions) 53.4 ± 0.2, 50.9 ± 0.4, and 44.7 ± 0.2, respectively. These values were derived from the median dry masses of populations of virus particles, measured by an electron microscopic technique. For all three viruses one protein, of around 40,000 daltons, comprises over 50% of the viral protein mass. Approximately 310 copies of this molecule are used in the construction of LPP-1M, 290 copies in LPP-2 and 460 copies in T7. The blue-green algal viruses differ from the bacteriophage in utilizing a second major structural protein in building the viral shell. Most other viral structural proteins are represented by no more than about a half dozen or a dozen copies. There appear to be no proteins represented only once in any of the viruses examined.