Abstract The conformational energies and electronic structure of the dipolar ion of phenylalanine were calculated by the CNDO/2 and extended Hückel molecular orbital methods. In extended Hückel calculation, Cusachs and Mulliken-Wolfsberg-Helmholtz (MWH) methods were used to evaluate the off diagonal Hamiltonian matrix elements. According to the CNDO/2 method, two stable rotational isomers exist: rotamer B (where the phenyl group is between the hydrogen atom and carboxylate group) and rotamer C (where the phenyl group is between the ammonium ion and the carboxylate group). Rotamer C is 1·6 kcal/mol more stable than rotamer B. The extended Hückel methods gave three stable rotamers. According to Cusachs' method the stability decreases in the order: rotamer A (where the phenyl and carboxylate groups are in anti configuration), rotamer B, and rotamer C; the MWH method gives the order as B > A > C. Results of nuclear magnetic resonance spectroscopy studies agree with the conformers suggested by the CNDO/2 method. The results also suggest that the stabilization of rotational isomers of the dipolar ion of phenylalanine is primarily due to electronic effects.