The structure of infectious bovine rhinotracheitis virus was explored with the negative contrast technique. The virus was propagated in bovine testicle tissue culture cells. The essential components of the virus particles were: (1) the core, (2) the capsid, and (3) the envelope. The core measured 945 A. The capsid consisted of 162 capsomeres and had an average diameter of 1085 A. The capsomeres were arrayed in a highly regular pattern characteristic of 5∶3∶2 axial symmetry. The envelope varied in size but had an average diameter of 2000 A.