  We report the discovery of a new spin glass ground state in the transition
metal monosilicides with the B20 crystallographic structure. Magnetic,
transport, neutron and muon investigation of the solid solution
Mn$_{1-x}$Co$_{x}$Si have revealed a new dome in the phase diagram with
evidence of antiferromagnetic interactions. For Mn rich compounds, a sharp
decrease of the Curie temperature is observed upon Co doping and neutron
elastic scattering shows that helimagnetic order of MnSi persists up to
$x=0.05$ with a shortening of the helix period. For higher Co ($0.05<x<0.90$)
concentrations, the Curie-Weiss temperature changes sign and the system enters
a spin glass state upon cooling ($T_g=9$ K for $x_{Co}=0.50$), due to chemical
disorder. In this doping range, a minimum appears in the resistivity,
attributed to scattering of conduction electron by localized magnetic moments.
