Phage phi29 is a nonisometric virus producing several types of morphological variants in normal infections. The study of these variants by electron microscopy, and their comparison with those from T-even phages, suggest that the capsid of phage phi29 is a prolate icosahedron. Phage phi29 capsid consists of a major protein, p8, and an additional protein, p8.5, making up the fibers. We have determined the number of subunits of each structural protein per viral particle taking into account the phage molecular weight (between 28 and 29.6 x 10(6)), the molecular weight of each structural protein, and the mass percentage of each protein with respect to the total protein mass of the phage. These values, together with the results obtained from chemical crosslinking of the structural proteins on the phage, suggest that the capsid contains protein p8 dimers clustered in trimers.