Abstract Following dissociation of bacteriophage ϕ6 nucleocapsid (NC) by EDTA, a particle composed of protein P8 and corresponding to the outer shell of the NC was assembled in vitro in the presence of Ca 2+ and Mg 2+. Assembly was obtained from soluble protein constituents frove 100 μg/ml and was optimal within a temperature range of 22–30°. Assembly did not require the presence of genomic RNA. Crosslinking results of intact NCs and in vitro-assembled outer shells suggested that protein P8 dimers are the structural subunits of the shell. Analysis of the assembly kinetics by electron microscopy suggested that ring-like particles of uniform size, packed in flat hexagonal arrays, are intermediates in outer shell assembly.