The anionic polymerization of 5-(N,N-dialkylamino) isoprenes carrying various alkyl substituents was studied in nonpolar (benzene, hexane) and polar solvents (tetrahydrofuran, dioxane, triethylamine). The microstructure is found to be strongly dependent on the bulkiness of the tertiary amino group. All polymerizations in nonpolar solvents occur by a 4,1-addition of the monomer to the anionic chain end. In THF no polymerization occurs. The results are discussed with respect to the stereoelectronic situation at the growing chain end. For monomers with the most bulky branched side groups (isopropyl, isobutyl, and 2,6-cis-dimethylpiperidyl) the addition is not only regioselective but also stereoselective, giving rise to the formation of stereoregular polymers with cis-4,1-repeating units. In the case of monomers with linear, less bulky alkyl substituents (e.g. ethyl, propyl, butyl, or piperidyl), the polymer contains a mixture of cis- and trans-4,1-units (until about 70%). In all cases the amount of 4,3-units is less than in the anionic polymerization of isoprene, despite the fact that the aminoisoprenes have a polarity comparable to triethylamine.