Glycoprotein gIII of pseudorabies virus is a major antigen found in the envelopes of virus particles as well as in and on the surfaces of infected cells. It is not an essential gene product for virus growth in tissue culture. In this report, we provide evidence that, although it is not essential, the gIII protein is required for efficient virus growth and that gIII mutants are quickly outgrown by wild-type virus in mixed infections.