The capsid protein of bacteriophage P2, encoded by the N gene, can assemble into icosahedral capsids of two possible sizes, with diameters of 60 and 45 nm, respectively. Only the larger capsid is used by P2 itself, but the smaller one is exploited by the satellite phage P4. We have analyzed the assembly products of gpN expressed in vivo from a plasmid, i.e., in the absence of any other phage proteins, and find that gpN alone forms closed shells of both sizes, although with poor efficiency. Coexpressing gpN with gpO, the putative P2 scaffolding protein, increases the efficiency of large particle formation. In contrast, introducing the sid gene by P4 infection stimulates the assembly of small particles. Our results suggest that gpO and gpSid act competitively with respect to capsid size determination. Furthermore, we demonstrate that gpN alone undergoes the normal proteolytic maturation steps, implying that gpN processing is either autocatalytic or mediated by a host enzyme.