We have measured the mass of the Z boson to be 91.110.23 GeV/c2, and its width to be 1.61-0.43+0.60 GeV. If we constrain the visible width to its standard-model value, we find the partial width to invisible decay modes to be 0.620.23 GeV, corresponding to 3.81.4 neutrino species. © 1989 The American Physical Society.