A novel catalyst composed of neodymium (111) isopropoxide [Nd(OiPr)(3)] and methylaluminoxane (MAO) was examined in isoprene polymerization. The Nd(OiPr)(3)- MAO catalyst proved to be highly effective in heptane even at low [Al]/[Nd] ratios (ca. 30) to give polyisoprene that possessed high cis-1,4 stereoregularity (> ca. 90%), a high number-average molecular weight (M-n similar to10(5)), and relatively narrow molecular weight distributions (M-w/M-n = 1.9-2.8). The catalyst activity increased with an increasing [Al]/[Nd] ratio from 10 to 80 as well as temperature of aging and polymerization from 0 to 60 degreesC. The polymerization proceeded in the first order with respect to the monomer concentration. Aliphatic solvents (heptane and cyclohexane) achieved a higher yield and Mn of polymer than toluene as a solvent. The M-w/M-n ratio remained around 2.0, and the gel permeation chromatographic curve was always unimodal, indicating that this system is homogeneous and involves a single active site. The microstructure of polyisoprene was determined by IR, H-1 NMR, and C-13 NMR. The cis-1,4 contents of the final polymers stayed in the range of 90-92%, regardless of reaction conditions, indicating the high stability of stereo specificity of the catalyst.