On quaternization of 10-ethyl-, 10-vinyl-, and 10-ethynyl-quinolizidine with methyl iodide, the formation of the cis methiodide vs. the corresponding trans methiodide increased according to the order of the bulkiness of the 10-substituent as CH₃CH₂>CH₂=CH>CH=C. The stereochemistry of the above methiodides was established. The N⁺-methyl signals of the cis 10-substituted quinolizidine methiodides were confirmed to appear at higher field than those of the corresponding trans methiodides except for the 10-ethynyl methiodides in their nuclear magnetic resonance spectra.