Substitution of magnesium for manganese in mixed perovskite La1−xSrxMnO3was found to reduce the electrical conductivity and the Curie temperature Tc. A remarkable jump in the resistivity ρ was observed in the 6% Mg-doped crystal with x=0.175. This sample remains semiconducting at low temperatures, contrary to the Mg-free crystal. The abrupt change of ρ reflects a first-order structural transition at a temperature Ts just below Tc. An applied magnetic field decreased Ts, leading to an enhancement of the magnetoresistance between Ts (0) and Ts (H). The microscopic properties of Mn3+ and Mn4+ ions due to Sr2+and Mg2+ substitutions were illustrated by 55Mn nuclear magnetic resonance study.