To explore the optimized colossal magnetoresistance (MR), i.e., higher MR with lower field, magnetotransport properties of single-crystalline perovskite manganites have systematically been investigated. Nearx=1/2with intrinsic instability of charge ordering (a1/1ordering ofMn3+/Mn4+), the one-electron bandwidth (W) is varied by reducing the radius ofR-site cation inR1−xSrxMnO3.ForR=Nd,the MR behavior is rather canonical, while forR=Sm,the field-induced nonmetal-to-metal transition of the first order shows up accompanying a change in resistivity by several orders of magnitude as a result of an enhanced antiferromagnetic interaction. ©1997 American Institute of Physics.