P22 is a bacteriophage whose morphogenetic pathway has been well studied by genetics and biochemistry. Electron cryo-microscopy has been used to study the three-dimensional structures of the Bacteriophage P22 wild type capsid and that of a mutant called sticky. The mutant procapsid contains a temperature sensitive mutation in the scaffolding protein, leucine 177 to isoleucine, which at 40_ C results in failure to release all scaffolding protein upon DNA entry. The sticky structure has been resolved to 19 _ resolution and agrees with the 22.5 _ resolution wild type procapsid reconstruction. The increased resolution of the sticky mutant structure provides visualization of capsomeric subunit separation and possible domain separation. In addition, finger like regions have been identified which extend from the inner coat protein surface towards the scaffolding protein core. A detailed analysis of the internal scaffolding protein core in these reconstructions has been performed to interpret the packing of the scaffolding protein within the procapsid. Based on this analysis we have proposed a procapsid assembly model where the scaffolding and coat proteins initially form an icosahedral lattice. The inner scaffolding lattice is then disrupted by the addition of filler scaffolding which completes formation of the inner scaffolding protein core.