Measurements of the electrical conductivity, σ, and thermopower, S, of liquid MgSb over the entire composition range are reported. A deep minimum in σ exists at c s, the stoichiometric composition (Mg 3Sb 2); S changes sign abruptly at c s also, being <0 on the Mg-rich side and >0 in the Sb-rich region. These results are analyzed in the light of a recent model for liquid semiconductors due to Enderby and Barnes. It is apparent that a rigid-band model is inadequate for this liquid, but that the gap in the density of states which promotes semiconducting character near c = c s closes with changing c. These conclusions are also consistent with measurements of the optical properties of amorphous MgSb.