The cryo-EM maps of HK97 Prohead II (PII) and four expansion intermediates (EI-I, EI-II, EI-III, EI-IV) are shown with a map of the mature Head II (HII) particle, represented at a resolution slightly greater than EI-IV. Underneath the EI-IV map is an electron micrograph of frozen, hydrated EI-IV particles used to calculate the map. Underneath the Head II map is a negatively stained electron micrograph of fully assembled HK97 viruses. Head II represents only the DNA containing "Head" portion of the entire virus. During the capsid maturation pathway, immature Prohead particles are observed to transition through at least four distinct expansion intermediates before achieving the final Head form. This "expansion" process results in a doubling of the interior volume available to hold the dsDNA genome, and a progressive formation of chain mail cross-linking architecture between capsid subunits. The cryo-EM image of EI-IV was provided by James Conway and the negative stain EM image of HK97 virus was provided by Bob Duda.